I've been super impressed with the AT50's, I preferred them over the 550's in the listening booth. Never heard the hd25 but I have a bunch of senn's and I way prefer the AT50's for electronic/bassy music even over my HD500's. the senn's tend to be warmer, the soundstage on the AT50's is really cramped, but the detail/clarity is stunning and with an amp (e17 here) they have great punch and the stage opens up a bit. I've taken mine on a few long trips and the durability has been great, plus the cable is replaceable. For the price I've never had cans I was more impressed with plus the noise isolation has been great even on 14hour plane rides
The HD25 is a complete departure from the typical Sennheiser house sound. More of a closed Grado - aggressive but very musical sound sig. I would say that it's a little more balanced, more refined, and faster than the M50s - however the M50s don't sound as congested as the HDs. For portable use, I tend to take the HDs over the M50s each time though. Better isolation and an addictive sound sig that's fitting for noisy environments.