I agree with spaceatl on the headroom for dynamics with clean tones. The DT50 wins there for sure.
My preference overall is the DT25. It weighs less, still sounds great, and does fine for all the gigs I do. I play a regular rock gig on saturday nights with a 4-piece band (2G,1B,1D) to around 100-150 people on a somewhat small stage. I use in-ears with all the other gigs I do, so there's no need for a larger amp.
If I was doing gigs on a larger stage w/o in-ears, I would definitely consider the DT50. I'd probably just end up adding an extension cab to my DT25 though. I guess I'm more about portability these days. I use to play with a guy who would bring 3 different amps to a show (2 heads w/4x12's, and one 4x10 combo) and 5 or 6 guitars just to get some exact sounds. Talk about a logistical nightmare. It was always interesting to watch him spend an hour setting up, tuning, troubleshooting the hiss/hum, and tweaking. Then, he'd be stuck in a 2'x2' square because he was surrounded by so much gear. ...Anyways, I'm sure that was a factor in my movement towards shrinking my rig.