huge advantage to the 31s batts is that you can get them at just about any truck stop in the country 24/7. if you travel alot, being tied to brand x battery because of reputation or warranty is totally useless because Murphy will dictate your point of battery failure and it WILL be 1000 miles from the nearest authorized dealer of your platnum plated megaschmaltz special , leaving you holding the bag as you hunt fr a new batt on the spot.
When you have 31s batts, they are generally fairly cheap, around 80-110 bucks for a very high amp batts with an insane reserve capacity ( which is the important part anyway ) and with the ease of availability, you arent going to fuss over brand etc if one poops out on you and you need to use it as a core. swap it out pay the man and go instead of winding up in an ordeal trying to find some fancy batt that dont exist for a 5 state area