Swerve: Belts or Chain?

[Qbot2640]

We've got a very young and energetic team (including a freshman programmer), and a new milling machine...understanding that this process may take several years, we are ready to start experimenting with swerve. Our plan is to make four modules and start playing with the programming and implementation.

A couple questions:
(1) The 15 tooth miter gears I'm finding on most robot supplier websites seem way too small to me. It seems to me, as I start to draw it all out, that a larger driven gear and a smaller pinion would make the geometry better - and accomplish a part of the speed reduction in the process. What experiences have others had here?

(2) I'm leaning toward using belts, but notice that most systems use chain. I definitely have a bias toward belt - I'll admit that, but what are the true pros and cons (other than the obvious, that is)?