Personally, I go with AGM. They're tried and true and I know them, and they can be found anywhere. Can have tested and charged anywhere with trickle charge on it, no need for a special charger.
Others swear by lithiums. They've come a long way, I'm just personally not ready to trust them. Plus, my bike is older, there's no official lithium made for it from the Moco or from any brand I have heard of and trust. There are lithiums available, just from brands I never heard of. Plus, electrical is my weakest area, and I dunno how my older charging system will do with lithium or if lithium needs a special charging system or something on older bikes that don't have a BCM (operating under the presumption maybe a BCM might adjust the input, if the lithium maybe needs different charging from the system? But I don't know about that either, too many "maybes" in that, just guessing possibilities).
Though any time I change the battery, because it sits down in the oil tank on my softail, I always debate about giving lithium a shot trying to lift that AGM out with just my finger tips, or trying to keep it lifted up enough with one hand to attach the positive cable. Them things are heavy when you can't get a good grip on them.
Not sure what the cost per year is between the two. I know lithiums were expensive a couple years back but haven't looked at them in a while.
I only get about 3 or so years as well. The climate is just too hot here for them to last much longer than that.
