I think there is no similarity and comparison cannot be done for those 2 dramas. Coffee Prince was a personal fave of mine years and years ago, but My Mister definitely wins for me.
Coffee Prince was a trendy drama with its interesting plot and very much hot chemistry. If you saw it way before (when it aired, or just a year or two removed from airing in korea, like I did) then I am sure you loooooove it the same way I do. So when you recall it it, would put a smile in your face because it hits the right spots for romance (and bromance)with its squeal wothy moments. But if you were also like me, who re-watched it about 3 years ago, the narrative and story itsel feels dated. Its great but i dont think it will thrive as an IT drama with the test of time.
And with My Mister, well I simply call it a masterpiece. Its probably more heavy than tthe usual trendy dramas right now. There is something really awesomely brilliant it its poignancy for life, down to earth in its visual, basic in its grandeur, humane in its sacrifice, somber in its bliss, compelling in its pain, poetic in its romance and many many many other dazzling awesomeness. It touches you in all the right moments and at right places. I think it would totally go down into one of the best drama of all time, in any genre, any country. (Much like 1liter of tears, sandglass, nirvana in fire). This drama will stand the test of time in its awesomenss storytelling.
But I do confess, I am probably way biased, because I am a huge fan of Kim Wonseok PD. His dramas Misaeng, and Signal are both my all time faves. And I do give credit on how awesome this drama is. The actors/actresses were sooo awesome, but Kim PD I give hats to you for this drama which is yet again awesome and brilliant.
But to each his/her own. As long as we enjoy both or one of these dramas, isnt that amazing in ts own....?!