As a general purpose backup supply - the XP1 is a better bet as it has more Output voltage options. Expensive - Yes and no. A decent LiFePO4 5aH starter (4S2P) battery will set you back about 80Euros just for the cells.
Personally I'd rather get a small 2.5aH LiFePO4 battery wired permanently in parallel and get some 5V Solar assisted charging battery packs for cellphones - cameras etc.