I am a certified poker dealer
and have investigated all the plastic cards on the market (as far as I know).
Here are my findings [note that the two main brands are Kem and Copag]:
Kem: the best of the best, IF you get a deck that isn't "bowed"-- sometimes the deck has a bend, either, down or up (down is better if you have to pick, as you will be riffling the deck "up", which should compensate in the long run; but you don't get to pick, the cards come as they are). Highly recommended with the caveat of getting a good deck.
Copag: the best as far as price to value; best consistency
in that I have never gotten a set up (two decks, diff. color backs) that has been bowed one way or the other (vs. kem). Print/color is the weakest of any. Highly recommended as an into plastic deck. You are guaranteed not to be disappoinited (vs. a perhaps bad Kem deck), but they are not the "best".
Gemaco: I found these to be a bit too "stiff" for professional dealing. If you shuffle/riffle your cards like most home games do, I am sure they would be fine. Price might be a consideration, as they are less than Kems. Not recommended.
Del Negro: by far the stiffest of the brands, almost unusable with the professional riffle. Good color/ink. Same consideration as Gemacos if you are not using the professional deal/riffle. Not recommended.
A-Plus: I would suggest these as the best "off" brand plastic card (Kem and Copag being the established). I thought the print on these was superior to the Copag (but no one can compete w Kem's ink job). They are slightly stiffer than either of the two main brands, but not so much so that they are bad (vs. Gemaco and Del Negro). Recommended.
Hope that helps....