The rx120 is a low speed rad... it works great in it's ideal application, but may not be enough for a high clocked 2600k. Your 2600k isn't high clocked so you'll probably be ok. Another problem with thick 120mm rads is you will run out of space inside the case to house the rad. If you don't mind you can run one fan inside the case and one outside, follow your imagination.
The MCR120 isn't quite as good as the RX120, and the MCR120XP will require the same high speed fans as the GTX... ANother thing, a GTX120 with very fast fans can dissipate a crap ton of heat, which not many other rads can do.
I dunno, for me the question comes down to your noise tolerance. High or low? That will dictate the relative performance of your loop. GTX level noise of RX level? Which is your poison?