Not only is a group 31 larger, it's also heavier. It's difficult for me to get my 31 out of the basement. I don't want to do that again.
I've had good luck with a single group 27 Wal-Mart 27DC for $80. I get 5 years out of one with proper maintenance. We will sometimes overnight without power and this has served us well. We can watch satellite TV for hours on a 100W inverter and run the furnace for a few hours if needed.
Your best battery selection really depends on your expectations and how you plan on using it. Do you want it to power your propane fridge and lights for a few days or run your microwave?
Battery maintenance is an important part of how well your battery performs. My WFCO charger will kill my battery if I let it.