There's an interesting trade off, here, though. If your hands become more naturally curled, even if you lose all or most of the strength in them, the tendon contracture itself will give you a functional ability to hold things, strength. If you fight to the last moment to keep your hands flat, when they don't have strength left, you need a u-cuff or similar tool to get anything done with them. So I think progression speed has a lot to do with that decision. I doubt this would be useful for someone who is going to lose all their arm strength in fairly short order, as in many cases of ALS, but for a slow progressing PLSer it can be a lot more meaningful.
There's a classic story told of a lunch given between two paralymic events, where the athletes didn't have most of their stuff with them. The quads with curled fingers threaded the forks between their fingers and ate away. At least one of the ones with flat fingers was caught out in the embarrassing position of not having his utensil cuff. Curled fingers can be an assistive device that is always with you.
Flat fingers, of course, are considered more aesthetic, so there's that. Also, it makes it easier to get gloves on.
I went with the "quad claw" plan, and even with just wrist extensors, it has been a big help over my quad years.