What are the pros and cons of prefab granite vs. custom slab?
The big difference as you mentioned is price.  The seams with the pre-cut pieces should not be an issue (assuming the installers are good at what they do), the more noticeable factor will be the difference in colors and patterns.

If the entire counter is cut from a large slab, the colors and patterns will be consistent throughout the entire counter.  If the counter is assembled with pre-cut pieces, you may notice differences in colors and patterns from one piece to another.  The choice here, however, would come down to preference.  Some people might prefer the look of one method over the other, so it's hard to say what you'll like better.

I would find out if they have a show room or can provide pictures (from previous jobs) of the different methods, then you can decide which you personally prefer (while keeping your budget in mind).