Let's do a proper comparison. (Just tried to put a 'k' at the beginning of that word, to try to start a thing, but autocorrect killed it and I'm too lazy to go type it again.)
Bugs: Way more bugs in a Tree Fort. Blanket Fort takes this one.
Climbing: Way more climbing in a Tree Fort. Blanket Fort takes this one.
I'm Afraid of Heights: Blanket Fort
Coziness: Unless you set a risky fire in your tree, a Tree Fort will never be as cozy. Blanket Fort.
Access to Fridge and Television: Blanket Fort.
Time to Construct: Tree Fort - weeks if you ever finish. Maybe even months or years. Blanket Fort - eleven to twenty-three minutes. Blanket Fort.
Feeling Like a Kid: You could definitely feel like a kid in a tree house in theory, but as an adult, the inconveniences makes it feel more like camping somewhere where you have to make a perilous descent just to wizzle. Blanket Forts make you feel like a goofy bastard immediately. Blanket Fort. (You could wizzle off the side of the Tree Fort, which would be fun, but you couldn't drop a twosy this way. It wouldn't be proper.)
Nookie: You're likely to get some in either arena, but more likely where there's a proper roof, door, pillows and blankets. Blanket Fort.
Woodiness?: Just trying to come up with something that the Tree Fort can win for consolation, but man, in my living room there's a wooden coffee table, wooden TV stand, I think maybe even the house is made out of wood. Let's call it a draw.
Short Answer: For lifting your childlike spirit in a fast and convenient manner, Blanket Fort is the clear choice.