I own the TF10s and I've had plenty of time listening to the M50s from a friend of mine
I think the TF10 beats out the M50s in every aspect (as they should) except for comfort. The TF10s are clear, have great detail, an open and airy sound, and great soundstage. The bass isn't boomy but it is punchy and has great control, vocals sound bright and alive, but instruments like violins (anything classical) sit at the very bottom of the the V-shaped freq. response so it does sound very recessed. Highs have great detail and extension but can be a little sibilant on some songs.
The M50s have a very average soundstage as most good reviews will note, they have a good amount of bass too which is a little boomier but I think lacks a little control that the TF10s have. And the mids and highs don't even compare.
The TF10 is notorious for being uncomfortable though so the M50s win in that regard (theyre also built like tanks). If you continue down the road of Hi-Fi and end up getting an amp, the TF10 is a better long run investment because it actually does benefit some from an amp (I have Fiio E10) whereas the M50s really don't.
If you wanna know more about the TF10s, I have an in depth review on my profile.
I debated getting the M50s for awhile for their bass, but after doing research and digging, I'm going to lean towards the Ultrasone HFI-580. Especially now that the M50s are so expensive compared to what they used to be..
As for burn-in for the TF10: People always argue that since the TF10s are balanced armature, they shouldn't need burn in, but when I got my pair they sounded so dark and congested that I almost immediately returned them. Fortunately I gave them the benefit of the doubt and let them burn in for 20 hours before listening. I decided it was a good time to see whether I could debunk the myth of burn-in or not and I actually timed out exactly 20 hours while they burned in with pink noise. During this time, I did not listen to them at all. When I came back to them, the sound opened up much more and they had the open, airy sound that they are well known for. I love my TF10s and I am a firm believer in burn in (though not every pair of headphones may benefit or benefit a lot from it)