[Oblarg]

Some thoughts, from my personal experience:

Belts are more sensitive to C-C distance. Chain is more sensitive to alignment. I've never once, in my many years of FRC, seen a timing belt jump off a pulley (exclusively using flanged pulleys, admittedly, has helped with this). I've seen chain jump off not-quite-aligned sprockets more times than I can count. However, I've seen plenty of not-quite-tight-enough belts ratchet, whereas chain will only ratchet if it's basically falling-off-loose.

Complicating this fact for chain is the tendency to stretch, which is not a problem with belt. Once a belt is properly set up, you can forget about it. Chain, not so much.

The upshot of this is that chains will (almost) always require either tensioners or adjustability at one end of the run. On the flip side, if you don't have the machining capability do to precise C-C distances, so will belt.

Belts are much, much quieter, which is a bigger benefit than you'd think.

Chain can be put on in-place without moving a sprocket, via. masterlink. Belts cannot.

Chains can transfer a much higher maximum load, given the size of belts used in FRC. If you're limited in your pulley diameter or belt width, or if you just have a mechanism with whopping forces involved, this can matter.

This isn't rigorous at all (so take it with a grain of salt), but belts seem to be less-prone to bizarre one-off undiagnosable failures than chain. A fair number of times I've seen chain fall off drives with no visible faults, with no hint of the cause and no subsequent failures despite nothing being changed. I have seen a grand total of one belt failure in my entire time in FRC.

My teams tend to use belts, because we value reliability and belts basically never fail.

On #25 vs. #35 chain, my advice is to avoid the former unless you are certain of your team's machining precision. As I mentioned, #35 chain is already more sensitive to mis-aligned sprockets than belts - but #25 chain is *far* more sensitive than #35 chain, still. If you cannot align the sprockets extremely well, it will fail. My teams, which machine almost exclusively with a chopsaw and a drillpress, have abandoned #25 chain for this reason, and we do not regret it one bit.