depends on how you're planning on driving it. If you plan on running it at the drag strip frequently, and not doing as much driving around town/highway.. then go for the 3.91's.

If you're looking for a good 'compromise' gear ratio, that will give you a good ratio for around town/highway driving as well as decent off-the-line performance, go with the 3.55's. That's what I've got in mine (not that I've ever driven it, but that's what I've got)