I have both realistic and non-realistic toys, and I find that the "heads" of realistic toys provide more stimulation for me most of the time. However, there are times when the shapes of my non-realistic toys are very stimulating as well. Guess it depends on the actual toy itself and my preference that day
I'd say I prefer my toys to not look too much like a dismembered body parts about 70/30. Sometimes a realistic shape and feel is nice (it seems to be what we are designed for, after all) but I shy away from realistic flesh colors (Goodfella only comes in flesh tones, though, so I got the one that matches me).
I agree with kristi, I like them both! The realistic ones definitely add more stimulation for me though, on those lonely nights! IMO variety is a good thing to have. Then you can pull out whatever you are in the mood for!
I have both, each type fulfill a need. My b/f initially did not like realistic looking toys but he got over it but as a result I have a collection of "vanilla" toys such as the VixSkin Raquel which I still so much enjoy.
Another thing to be consider would be maintenance. It seems that the majority of the realistic toys are higher maintenance, to maintain their texture. I personally don't have a problem with that, but if you don't have a lot of time to take care of them they probably won't last very long.
I also am becoming a huge fan of silicone toys, oh em gee.