I do not like mixing with headphones at all it just seems so unnatural, but this may depend on the kind of music you are mixing. I have rokit-5's and love 'em, nothing like mixing with a solid set of monitors. The m50's are great too but just for personal listening and as monitors for recording in my opinion.

Mixing with headphones is okay, but mixing on monitors will always be better IMO. One thing to keep in mind when using monitors is that it's very important to have them arranged properly and most of all have a treated room. Even if you can't afford professional treatment or even acoustic foam, it doesn't hurt at all to hand some thick blankets up on your walls and stuff.

honestly if you have the opportunity to own both, you should not choose between the two. What is arguably more important than having the best pair of headphones/monitors, is having as many references as possible for your mix.

Second this! It's always important to have as many reference points as possible. I keep a normal pair of speakers connected to my computer just to make sure it sounds good on consumer audio devices, and I always try out a mixdown in my car too.

I just got krk 5's because I don't like having headphones on for long periods of time. If you're fine with wearing headphones all day, then put off buying the monitors until you can treat your room.

I think getting a pair of studio monitors might be a good choice for you, as a sort of compromise. I know ATH m50's are often advertised as being studio, but they're actually a fairly long way off from having a flat frequency response. That's not to say they're bad headphones, just not the best for mixing, as was said before.
And as was also said, getting monitors in an untreated room is less than ideal.
So studio headphones will make mixing easier for you but also won't require too much setup, and aren't too expensive (you can get some models for less than m50's, and loads in the same price range). That's where my vote goes

(I personally use a cheaper version of Brainwavz HM5/Fischer Audio FA-002's. They're pretty nice and are really, really light and comfortable, so i don't have to worry about wearing the for hours on end)

Headphones are great to hear details with, but hearing music on monitors in a room helps you understand the physical properties of sound and how it spreads and bounces.

This. Unless your binaural perception is immaculate, you're probably going to find yourself with a lifeless mix that has everything either hard-panned, or mono AF.

Depends on YOUR preferences at the end of the day. Just keep in mind monitors allow you to visualise and appreciate the placement of instruments across a 180 degree spectrum (from L to M to R) much better than cans.

They both have their advantages for sure! If you are on a budget go with headphones but if you have some cheese to spend try both. I have headphones and monitors and I never stop finding a better way to mix on both. Remember there is no right or wrong answer it's going to come down with what are you comfortable with