Hi There,
I own both the M80 and HD25. I love them both. However if I really had to pick just one of them, I'd still go with the HD25. I travel very frequently on trips that last 2 weeks or more and I've been rotating between the M80 and HD25. This gives me a chance to evaluate both these cans under travel conditions and also forces me to live with just one of the cans over a slightly longer period of time with no temptation to switch.
While the M80's are built to take a beating, the simple and understated ruggedness of the HD 25's win out when you're fumbling with your carry-on going through customs and getting on and off transportation. I also find the HD25's are little more rewarding to listen to.
However, all of the above also depends if you are planning on running an amp with your portable setup. My source is an ipod touch or an iphone and I almost, always carry a portable amp with me. If you are planning on playing your tunes directly out of an idevice, I think the M80 is a better choice.
Best,
Dan