Usually they're one or the other, but some do do both. It's hard to gauge numbers because they vary by geography, and by business type. Waterbased inks are more common outside of the US; I suspect (but don't know) that they'd also vary by geography within the US (so they'd be more common in California for example). General screen printers (as opposed to garment only printers) are more likely to use a variety of inks.
Water based ink dries in the screen unlike plastisol and is supposedly "environmentally friendly" (search the forum for the great debate on this ). Water based inks are better for printing over seams and generally need discharge agents for printing on dark garments unless you have a discharge underbase. Waterbase has its advantages as does plastisol. Waterbase inks are advancing a lot from what i've been reading recently and will definitely continue to gain popularity. Waterbased inks have little to no hand which make it a staple for high fashion clothing lines these days.
Water based ink is PVC free and needs to expel all the moisture during curing to be completely cured. I'm not the all encompassing guru of waterbased printing, but mine are turning out great. Search for some of the posts by Dan @ forward printing or Brent @ Vacord.com for more detailed info..
i only use water base inks becouse it air dries you dont need convayor oven or flasscure i just use simple hair drier or just hang it and dry by it self easy to wash,
I understand all that, but I am still not understanding the deciding factors. A customer comes in and wants there logo on a shirt I doubt people ask "do you want plastisol or water base"...most people on the forum dont know the difference let alone the customer. There has to be some factors for one or the other and it cannot just be soft hand, because I would not think there would be to many customer that want a big "patch" print on there shirt...most would want a soft hand/comfortable feel.
Just as a hypothetical question. Say I have a small shirt run (20 or less shirts) one color one sided job. I don't want to fire up my large oven for 20 shirts, could I use water base ink and hang them on a clothesline to airdry? Would I basically use the same equipment and chemicals for the process as I am using now for plastisol?
I guess I never really thought of the air dry advantage until I just read some of these posts.
I hate firing up the oven for small runs.
I don't want to fire up my large oven for 20 shirts, could I use water base ink and hang them on a clothesline to airdry?
You can do that to air dry them, but they still need to be heat cured. So if you don't have a looming deadline you could hang them to dry, set them aside, then cure them along with other prints later. Or you could cure them another way (heat press for example). Or you could use an additive in the ink so they do air cure (but there are disadvantages to that).
Quote:
Originally Posted by auggieboy
Would I basically use the same equipment and chemicals for the process as I am using now for plastisol?
Basically, yes. Depending on the emulsion you're using, you may need a different one (but there are emulsions compatible with both inks). And other than in screen making there aren't really any chemicals involved with waterbased ink (no ink degradent, press wash, etc. - just use water).
Water based/Plastisol, or both 
