I prefer DMing at the moment, mainly because my current group is fairly new to, and less invested in the game -- they've run some fun stuff when they've taken the reins, but they tend a little too much toward the random encounter... (Also, there's the problem of knowing the rules, or more importantly having enough of a feel for the game and its mechanic that you can eschew the rules when necessary.)
Playing for someone who DMed with the same level of investment that I do could be fun -- although I have a very strong distaste for games that the DM leads (which is to say, traps you in a particular plotline, doesn't let you explore other things). This can often be a source of frustration for my players, because I tend not to give them anything more than the hook -- the rest they have to dig up themselves. But IMO, the really fun thing about RPGs in general is the degree of freedom they give the players -- and coming up with situations that require clever, inventive solutions is fun for me, especially as I watch it play out (from the internal chuckle of watching them struggle with it, to the "oh, I never thought of them doing that.")
Basically, when I DM, I come up with a scenario where I can't see any course by which the player's could "win" (while keeping CRs and everything else balanced enough that I can tell they have a chance)... From that point on (as in, once the game starts), I try to keep the players running the game -- and just try to intuit the results of their actions the best I can, while interfering (making monsters enemies behave out of character, throwing in arbitrary NPCs to further the plot, et cetera). Since I don't see myself as actually "controlling" the world (just coming up with it, and trying to see what its natural course would be) it becomes more like watching a movie.
Which is why I prefer it to playing, I guess... Short of a really good DM, it's harder to really visualize it cinematically as a player. I'm not sure how well I get that sense across as a DM, but whatever comes across, my players still enjoy the game. So I prefer DMing, for now.