Tiny Chat People

Aside from the Spit thing, Rein seems to be better than Grind in every way that matters to us personality-wise. Where Grind seems likely to get mad at us whenever we fuck up, possibly even to the point of deciding it would be better to keep us from being incarnated (which is arguably the correct position, though that's beside the point), Rein is a literal god of patience, and would likely be able to help us suck less. Ideally we would ally with everyone, and no cats would have to die, but thanks to Grind that probably won't be happening. Unless Grind has a really great reason to value keeping Rein from winning over keeping all the cats safe, then pragmatically and morally, Rein is the better option.

Of course, there's no reason not to stay at least nominally neutral for as long as possible.