mobile

I would like the maximum possibility of custom firmwares, etc. Which carrier's version has unlockable/exploitable bootloader, and so on, such that I can reasonably expect to be able to install custom firmware for a long as the device is supported by devs? I've looked through a bunch of threads and nothing is really giving me this info, as a person shopping. It looks like Sprint has the lowest (subsidized) price, though T-Mobile tends to have the most "open" devices. That said, I have a Verizon account already.

Which carrier's version should I buy? I don't want to be stuck with something like Safestrap (no offense to Hashcode, his work is awesome!). Is there even an option that answers this question the way I want?

Related.... if anyone knows, how do the sizes compare? I have been using a Nook HD+, which is supposedly 8.9 inches. Is the 8.4 going to be most similar to that? It appears to be narrower from pictures, but I don't want to jump up TOO much in size, either. Anyone who's used both, is the 10.5" too much tablet?