Now that nBoot is here, and I have the ability to replace my apparently defective Nspire for potentially any hardware revision, I'd like to know what to pick if given a choice.

Huh? nBoot let's you do that? Or are you just saying that you now have the ability to buy a new calculator?
Anyway, if you mean you could buy a new calculator, I would go with the revision A; it's before TI started changing all the parts for cheap garbage, and nLaunchy supports it. If you want the latest parts possible, you could go with the revision I; it's the last calculator before they disabled booting the 3.1.0 boot2, somehow. If you don't care about nLaunchy, you can use the revision V, which was the last revision before they changed the screen and made most ndless applications incompatible.
If you mean that nBoot let's you do that, then all the things I said about hardware (the screen, the parts) don't matter. I'm not sure what changing the revision via software would really do for you, most revisions are hardware-related (except for the boot1 updates, but that's read-only [without heavy modification]).