From what I can gather, it seems the features of each one are pretty similar. The main differences being if you can reprogram the flash chip externally or not and how easy it is to solder/wire it in. The actual firmware is whatever you want to put on it.
If you're not going to be writing your own firmware then the ability to easily reprogram the flash memory with an external connection isn't going to be that important for you. It's only used if you write a dud firmware into it and then need to recover. The XBox itself would be able to reflash normally.

- Trevor