I have yet to develop much in Chicken, but I've looked at it closely. It works on a different philosophy, one of more practicality (and less elegance). Because of that, it's matured a lot, and it has tons of libraries which is a huge benefit.

Gambit is more lean and portable, good for uses like compiling to the iPhone (that would be much harder for Chicken).

That said, I would probably use Chicken for my next project.