This question surfaces time and time again, and the answer is simple on one level, and complex on other levels, depending on what your motives are. So, here goes.
Juicing has been around forever, as in hand-wound mangles for crushing sugar cane, and of course citrus juicers.
And blending sees a long history with grating and mashing root veg for livestock.
But it started to grow in popularity around 100 or more years ago, as the vitamin content of foods started to fall, with major food production and chemical fertilisers appearing - yep, that long ago - but took a long time to reach the frenzied heights we now find it occupying.
Much of the effort of digestion (especially vegetables) comes from having to break through the tough cell walls - which is specifically designed to protect and contain the cell contents. We have teeth and jaws to smash them open, and then profound chemistry in the stomach, liver and intestines to break things down further, allowing the microbes that we house to reduce the resulting nutrients into compounds that we can actually make use of at the biological level.
The upside is that if we use a juicer to smash open cells, revealing their intimate contents, and then separate the cell structures out, we are saving the gut a lot of time, energy and chemical effort.
However, we are also exposing those nutrients to all the forces that their cellular prisons were protecting them from - microbes, oxygen, and even light - which all quickly degrade the usefulness of these nutrients.
So, if you're gonna juice, you need to make use of it before the rest of the environmental factors do.
To get around this, most fruit juice consumed has been juiced far away and pasteurised to give it some shelf life - possibly negating the whole operation, as the vast majority of viable nutrients will be lost to the heat of this process.
But let's face it, it is still better than a Coke !
Moreover, certain compounds are tricky to handle in mass amounts - such as sugars of course - which the body likes to receive in spoonfuls (like from fruit), rather than bucketfuls (once they're juiced).
Juicing fruits increases the availability of sugars to such an extent that the detriments of this can outweigh the merits of other nutrient availability.
This depends massively on the resilience and resourcedness of your particular sugar-managing system. Most diabetics find it too 'spiky' a ride, while the general populous still doesn't notice the 'hit' from a carton of clarified apple juice.
See the peaky excesses of sugars in the previous article here.
First lesson : Eat fruit ; juice vegetables
So, looking at vegetable juices rather than fruit makes the whole subject calm down a lot, as there is less of a sugar issue to be concerned with.
Plus, people tend to need to consume more veg, whereas most are content with at least some fruit (no where near enough, but we are still talking about the masses here). Rather than cooking every vegetable that passes one's lips, juicing makes them into favourable meals, as long as you can be bothered with all the washing up that a juicer requires.
However, we have lost the fibre due to the way juicers work, so the juice meal will need to be followed by another and another in fairly swift succession, as all the nutrients therein are easily and quickly assimilated, and there is no fibre to slow the process down.
A great way to massively increase one's vitamin and mineral content, whilst spending lots of time in the kitchen and shopping for endless quantities of veg.
Vegetable juice (especially greens) form the cornerstone of most serious health regimes and serious disease management techniques, such as Gerson Cancer therapy - which still heralds one of the highest success rates in the world (success meaning elimination of all traces of cancer and a full return to health - rather than simply a temporary mitigation of troubling symptoms, and predicted return), which seems to be the best most orthodox treatments can offer.
While it may seem closely related, as it is still smashing up plant matter with machines, this is a completely different game, due to the fact that nothing is separated and discarded from the mix.
In short, blending is simply replacing teeth by doing a mechanical means of cracking open cell walls to reveal the delicate contents within. Obviously if this is done in the mouth, then there is the necessary benefit of salivary secretions, which initiate some forms of digestion there and then.
Given that the smashing is happening in a glass jug, we simply have lots of corrosive oxygen being whisked in. If we add a little water to the mix, we do lessen this impact, as much of the air is placed with the liquid.
As with juicing, once the meal is created, the best thing you can do next is consume it, as the more time exposed to light, oxygen and bacteria, the more degradation will have happened - denying you those hard-earned nutrients.
Because nothing is discarded from the blended foods, they retain most of the fibre content, which leaves them just as filling as eating the foods separately.
So, blending is usually used as an attempt to change the form of foods, and mix of flavours that nature created. IE: We tend to make smoothies because we like this new flavour blend, or they fulfil a convenient method for getting certain foods inside us - superfoods, for example.
Whereas juicing tends to be more about shoehorning additional foods into the health-seeker's day.
Sometimes the juicing argument gets entangled with the need for additional fibre, from which it does not fair well, as it deliberately separates one from the other
But then, if you want more fibre in your diet, eat more fruit and veg - preferably raw.
But that doesn't stop you juicing too.
The Green Chemist
The best thing about juicing is as a health food shop. A multivitamin hit.
If you find yourself apparently deficient in any nutrient, simply find a decent list of veg/fruit that contain it, and start juicing - in a bigger way, the better - and go get another evaluation after a fortnight. If you're still deficient in it, then it is not because you are lacking it in your diet! It'll probably be something about your gut wall or metabolism.
You'll spend a lot on veg, but you will feel it in more than your pocket.
The point is that you cannot eat 3 meals a day and eat 50 carrots.
The point is that you can eat 3 meals a day and juice 50 carrots.
You will lose 5 carrots worth of nutrients in doing so, but that leaves you 45 carrots in the bank.
The big question then is...
Which juicer ?
Personally, I've seen them all come and go, and I understand the stats about how much better the juice is if it is cold pressed, slowly, in a shady kitchen, with a nitrogen-controlled environment, by virgins on a full moon. Full-on mastication !
Personally, I shy away from anything that takes longer to prepare than it does to consume.
And definitely if that includes another 10 minutes of washing up.
But that is apparently just me, as the top-end juicer markets are knocking them out at £300 to £1000 for the top of the range Norwalk.
I get it. With its sexy steel finish, 2 tonne hydraulic press, and pleasingly masculine submarine parts. The juice contains zero cell structure, has not been changed significantly by the process, and does taste like cream.
But truth be told, I'm happy with my £50 centrifugal juicer that comes out of the cupboard infrequently enough as it is...
But the set up, use and pack down is over in a flash, leaving me with a pint of juice that tastes good enough for my tastebuds, and critically took less time to make than to take !
I do always have the option of getting down my old (impulse-purchased) Champion from atop the cupboards - where he usually sits gathering dust like Wheezy from Toy Story - for the those rare green juice moments.
Despite being one of the best, he is nonetheless proving less popular than Wheezy...
And I've had his squeaker fixed !