I think 15 feet is fine.
I bought two nice cheap ones from Menards for like $50 bucks a each. I think they are 12 footers. They work great. I also got one of these for them from Cabelas. Not only does it add safety it allows you to get a steadier aim, or leave your rifle resting on when your sleeping. (Yes, I sleep a lot in my deerstand.) :lol:
Work great. I've shot plenty of deer from stands that were between 12' to 20'. Most of them were in the 15' height.
I've also shot lots of deer sitting on the ground. But I recommend getting one of those cheap foam cushions to keep your butt dry and warm. :lol:
For your 2nd question: if your too high it makes your angle higher which makes the vital areas on a deer very small if your a bow hunter. So some bowhunters don't go too high because it makes it harder to get a good shot.
For your 3rd question: I like ladders because their simpler to set up, just put up against the tree and strap down.
A hanging stand and ladder are fine also, but their more work, if your older or weaker it could be harder for you to physically hang a hanging stand by yourself.
I have a Loggy Bayou climber that I really like also. It's probably harder for me to set up a hanging stand then use a climber.
I really like ladder stands now though, since every time I go out it's just up and down the ladder. Versus a climber where I have to climb up and down every time. A ladder is also the easiest set up.