I would always given the chance go for a AGM battery, now I have one on my Ascot I get none of the problems I had with a more old fashioned lead acid ones and mine is only a Dynavolt which seems to be the cheaper end of the market. I also fitted a YTZ14 battery to my wifes MP3 scooter thing in place of the YTX14 batteries which for some reason just do not last, it was a bit shorter but still sat nice and secure and hopefully will last longer and do as well as the YTZ12 I bodge onto my SD which has lasted 6 years of barely being used.
I've found it's worth checking out dimensions of all available batteries because sometimes it's possible to fit a better battery than what is recommended.
I am a little confused about the battery fitted though, as the parts lists states that the original battery was a 31500-MK4-612 which is a YB12AB for which indeed a CB12AB is an equivalent Honda supply both (although the CB12 is much cheaper as you found) so Silvers and Wemoto are correct. The last two digits usually denote the way around the terminals are although I never understood the coding myself, always working from part numbers, but if A is one way then B is as I recall the other which corresponds with your experience. The battery never changed on XBR according to the parts list none should not have a YB14 on, they didn't even use that on the GB500. I have to conclude a previous owner did some modifications. I think you might need to look deeper but in the end it's up to you whether you replace it with one like the old one on it, or return it to standard or as I suggested see if you can find a sealed type replacement battery. Mind you that £28 YB14 sounds tempting.
