Mo is right the abs rims are lighter and also a stiffer rim (which will give you a little more corner speed), this is not always a good thing though. If the rim design on ABS plastic is crap then the rim can crack and break. If you are a good driver and dont hit much, abs is better. If you want more durable rims go with the nylon rims, they can take more hits. Heres a list of some good rims:
Kyosho 15 spoke rims: medium weight, good design semi stiff, nylon plastic. (recommended) works well on gas cars too
Schumacher 15 spoke rims: Medium weight, good desgin, very stiff, Some other material (recommended), works well on gas cars too
Yokomo abs dish wheels: Light weight, ok design, very stiff, abs, good for national events where everything counts.
Hope this helps