No big reasons, but maybe something to think about.
XLR-1 uses AAs, same batteries as D50. It's convenient when you don't have to carry different types...especially so if/when you are thinking about rechargeables and having to carry the chargers with you (traveling field work).
Denecke PS-2s operating time at 48V, 10mA per channel load has been tested to be 18 minutes. Users can probably give more accurate real life figures with different mics, manual states 4-38 hours depending on load. XLR-1 works for few hours, manual states 40 hrs for a 2mA/mic load.
XLR-1 uses good quality audio transformers for unbalancing the signal, the only proper way to do it. The audio circuit is passive. PS-2 has electronics to do the same.
XLR-1 mounts directly to the main unit couple of different ways if needed. Might be too big package anyway, unless you have large pockets. PS-2 is a separate box with no direct mounting, if not modified.
For me XLR-1 wins in almost every aspect. Comparing to the specs and prices of real pre-amps is not quite fair. Not to say that something like MixPre wouldn't be a nice companion to D50, quite contrary. But USED MixPre vs. USED XLR-1 would be more fair anyway, right?
If you need more clean and quiet preamp gain for quiet sources, none of the batteryboxes will do. If not, XLR-1 is better suited for D50 than any of the others, imho. You can prove me wrong, offering me the same specs and form with less price. DIY will do fine, been doing it for ages myself. I can tell you, this nut isn't that easy to crack though, not for cheap.