Amping will tighten up the M50 bass. If you are looking to save $ they are good choice.
HD25's are more portable and they will still sound better than the M50 unamp'd.
The problem with the comparisons on this thread is that no one is mentioning the version of m50 or hd25 they have. Old m50s are bloated in the lower bass, newer ones are more balanced. Old hd25s don't extend down to the deep bass while the new ones do. Check this out here for example...
My K550, has a MUCH bigger sound stage and deeper tighter bass over the HD 25, and it isolates very well. not as good as the HD 25-1 II but nicer than expected.
But I have only heard it amped through my Matrix M Stage