Hmmm warranty doesn't really mean a lot on back up equipment, as by definition it shouldn't be used much!.
However if you are using your back up equipment more than your main equipment then maybe your purchasing priorities are slightly mis-aligned
Imagine buying a Cd Player, keeping it hanging around, unused for 15 months and then the one time you needed it, it didn't work!

. So make sure that you at least run it for one night after purchase to ensure that it works okay!.
Brand and Model are not really prime considerations for back up purposes because ideally you should be going for years without any need for them....at least if you purchase wisely and astutely with regard to your main equipment in the first place, and of course have it flightcased and service it.