When I carried one, it was always a 12" version on either an M-16A2 or an M4. Mostly, my planned use is "because I can" and "it fills a hole in my NFA collection." I'm sure the most common round I'd fire would be TP, though I wouldn't mind having some of the other assorted rounds down the road.
Currently, I'm leaning towards a rail-mounted 9" version, which could be moved around among my various rifles, should I desire to do so. But, having never fired a rail-mounted M-203, I figured I'd see if the folks here had any inside experience which pushed them away from rail mounts. And, having no experience with the 9" version, I don't know if the shorter barrel would be a detriment to range or accuracy. (The most common host would be my 10.5" 5.56 AR, so a 9" barrel seems to be more of a natural fit to me.) If I do get one of the stand-alone mounts, it won't be right away. But, again, having never used one, I don't know if it'd worth the money, or which one would be more enjoyable and worth the time sleeping on the couch.