That's what I thought too, but evidently, the P bikes do that and orientation is not an issue for AGM batteries. This came up in another thread. Better check w/ your battery's mfr regarding installing it on its side - something that is not possible when buying it on ebay or Amazon. Personally, I shy away from off brands. Depending how the battery fails, jumping might not be possible (say an internal short).
W/ regard to Lithium batts, the advantages are obviously lighter weight, supposedly longer life. Corollaries to that are the bike will be lighter to pick up when you drop it, and it will accelerate faster
. Disadvantages are you might want/need a special charger for your battery that is comparatively expensive (and might be dedicated to that brand of battery), long term storage requires that you discharge the battery to about 60% charge (done by the special charger), cold weather starting might need you to turn the lights on for 30 sec to warm up the battery before it will turn the bike over. Better check w/ the mfr to verify that orientation is not an issue. I got a Shorai for my regular ole ST (along w/ their charger) and am happy with it. I have not had it for 5 years yet.
Check out batteryuniversity.com. This should make you a go to person on this website. Quiz tomorrow at 3 PM.