how bike is to be used will decide which battery to go with.
so far very impressed with Earth-X cranking performance. it delivers cranking amps as claimed.
but any battery with lower actual amp hour reserves like Earth-X ETX24 or Shorai LFX21.
there are situations where bike's alternator just is not given enough time to recharge battery back to full.
this is where Earth-X ETX36 or Shorai LFX36 with it's extra amp hour reserve shines.
BMW airheads with low 250 watt output (not K100), combined with long crank times, heated gear, lights and cold weather.
with all the extra drain, output is barely enough to keep from depleting battery further especially on short rides.
folks don't realize this and blame the battery next morning when bike will not start in 20f degrees. it would be good to use a Battery Tender Jr. (only) overnight to charge backup after a short cold ride with heated gear.
older bikes without fuel injection could take 20+ seconds of cranking when temps go down below say 30f.
unlike fuel injected modern bikes in proper tune, will usually start after a few seconds cranking even when cold.
if bike is to be used on long trips, including winter riding. currently the only LiFePO4 batteries I can recommend are Earth-X ETX36 and Shorai LFX36. if you are mounting battery in a non stock location. make dead certain no possible way for positive side to ground out. don't loose your bike to a fire. lithium batteries discharge at huge rates producing heat equal to arc welding.
if bike is to be used only on rides from home garage with a fully charged battery everytime. then a smaller LiFePO4 battery can be used. Earth-X has an internal BMS which gives overcharge protection to about 3amps. but my recommendation is only use a quality battery charger like Battery Tender Jr. that actually terminates charge after X volts is reached.
size comp... Shorai LFX36 on left, Earth-X ETX36C right