How many of the current battery boxes are really giving the true P48, if needed? I've been considering ART PhantomII as one option, but according to some tests it will not go up to full 48V. The XLR-1 specs state 48V (+/- 0.5V), although I have no idea if this is true. On the other hand, plenty of mics only need 24V and will regulate any higher voltage into that.
For me the bigger thing is the actual batteries used. D50 uses AAs, I have a decent charger for those and would like to use the same thingies for the BB also. This way my travel package would stay economical and as streamlined as possible. XLR-1 uses the AAs, whereas many others use the 9V batteries.
Since I'm found of recording low level nature sounds with D50, rather than loud music, the quality of pres come into play also. Simple BB might not be enough, although XLR-1 has at least the decent transformers for the proper unbalancing.