Out of those two I prefer the ATH-M50s. It has a much better and more realistic bass presentation (although perhaps a tad too strong), it's not quite as comfy but the seal and isolation was better for me. Both drive well from portable gear, but it sounds like you're not fussed about that. On the down side I find the M50s a bit muffled and the K550 a bit screachy in the highs, but they are both pretty well loved. For the budget you seem to be going for however I would recommend a couple above those two (in order of greatness, best first):
SoundMAGIC HP100
Beyerdynamic DT770 Pro
GermanMAESTRO GMP 8.35D