Which is better?

It really depends what you are doing and which quad core it is. I assume you are looking at a Q8200 vs an E8500? With that much of a clock speed difference it is really dependent on what you will be doing, if most of what you do doesnt scale well with more than 2 cores the E8500 is going to do better.
 


Yes. Although I do agree that speed to plays its part while rendering. So getting the quad here and overclocking it to say about 3.0 GHz (if possible) will fetch you more benefits.