I don't know what to choose. A bed with a desk (seperate) or a loft bed?
There are some units that have both! The bed on top, and then the desk area under. That sounds like it would work out Great for your needs:) Good Luck!!
Loft beds are great for creating usable space and storage. I built one for my nephew and under one end we put a corner computer desk and on the other end we made a media center. Everyone admires it. There are two issues with a loft bed... one heat rises and it tends to be warmer than the rest of the house unless you have adequate ventilation/cooling. Two, after a while people sometimes get tired of having to climbs the stairs before crawling in.
If space is a real issue, the loft bed will allow you to open up the floor for desk and other uses. The loft bed is difficult to make in the sense that tucking in the top sheet and covers would be difficult. But if you have a fitted sheet and a sleeping bag or comforter on it, you would just have to wash that bedding periodically and not worry about tucking in sheets as our mothers used to so meticulously do. There are bunk bed/desk combos available in catalogs and furniture stores that fulfill your needs. Not a loft, but a bunk bed might be just what you're looking for. If space is not too tight, I prefer the traditional bed and desk, as your bed always doubles for other uses (putting on shoes, folding clothes, you get the idea....). Hope this helps.