Chances are the prefabs might have more seams and may not be located in the best place to hide them. With custom, the seams are usually placed on a 90 degree angle back into the corner. This makes them less noticeable. Since these counters will be n the house for a long time and the diff is just $500. I would go custom from the start. You'll likely get a better peal product.

Personally, I have never heard of "prefab" granite slabs before. One of the main reasons that many have an interest in granite is because it is custom cut for your home. Did the shop tell you what the difference is between the two? In my opinion, if you're going to go with granite, you want it done right--because it's not an easy thing to fix once it's installed! I would go for the custom.

They're basically 9' predone with bullnosed edges. Like this http://www.sparkskitchens.com/Prefabricated-Granite.html

Going to with custom. Added another small workpace which would require another slab so the price gets even closer. I think they charge so much to install prefab to cover breakage.