I think XS is a 3 year warranty. I just got a D3100, because I could only use one battery. I had to cut the shit out of my stock battery location in order to squeeze it in though. When I get an M4a, I'll probably do a bank of the Duracells.
Group 31 will be a bit bigger if I'm not mistaken. You may be able to modify your vehicle to fit it, but not everyone wants to do that. You may be best off getting a group 34 if it fits in your vehicle. As far as a secondary battery by amps, bigger is better. I'd go with the group 31 for that.