Oh no, I think the HP50 and M500 are very different, I have them both, but I must admit I like the M500 more, why?
The M500 is much more comfortable, the sound is more dynamic and I like the rounded treble, makes bad recordings sound tolerable. The HP50 is more neutral and more "correct", but the M500 is more fun to listen to.
Soundstage is best on HP50, no doubt, which is the biggest plus to the HP50 compared to M500.
Build quality is much better on M500, really feels...