M50
-Pleather ear pads and closed back, ears get cooked after a couple of hours.
-Neutral sound with heavy emphasis on bass, go for it if you're a basshead and listen to a lot of electronic music.
598
-Light weight and velour pads make em super comfy and ideal for extended periods of usage.
-Open backed resulting in a better soundstage. Mids and highs are great but bass lacks punch.
-I'm usually not the one that prioritises form over function but these look like shit (imo).
They are very different from one another (open and closed back have their own pros and cons) and you really should audition em before buying to avoid disappointment.
You should also be looking at the DT990 pro which is another great pair at this range.