I'm going to agree with most of the other posters and say day bed. You don't have to get the extra bed underneath it, and the extra room is great to have. I've only slept on a futon once or twice, and found it lumpy. I've never been in a loft bed, mainly because I sleepwalk.
You also could just put your mattress on the floor.
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Futon: RUN AWAY! I slept on a Futon for about 6 weeks in Australia and it was uncomfortable as HE-double hockey sticks. Seriously. My back was not the same for about 4 months after.
Loft Bed: Kinda freaks me out. Don't know if I would like to be up that high.
Day Bed: Slept on one when I lived in Cow-town. It was good.
there are special risers you can get to put under your bed. I have seen them at linen's and things and Bed Bath and Beyond. I haven't used them but they look solid.