I've had top bunk for a VERY long time. I disliked that I couldn't get to my alarm just like that. I had to jump down and do it. Now, I can turn my lamp on whenever the heck I want :P
Actually, the top bunk should be the ONLY bunk... put a desk/bookshelf/file cabinet where the bottom bunk is, and build another unit just like that where the desk currently is... then both get a top bunk and their own workspace.
But the Top does have it's advantages too. \m/