MSI AMD Radeon R6790 OC or Gigabyte ATI Radeon HD6850

an hd 6850 will be better then oc hd 6790

You have already a gpu that is equal to one tier less than 6750 according to tom's hardware http://www.tomshardware.com/review [...] 107-7.html . Are you sure you want to do such a small jump from a little lower from 6750 to 6790/6850? In my opinion just for direct X 11 doesnt worth it.
Since you havent change a card for more than 4 years, why dont you wait for 1-2 months? there are gonna be new mid-range gpus that they will be far more powerful than than 6xxx (due to the change from 40nm which 6xxx were to 28nm which 7xxx will be). So by waiting 1-2 months you might get a possibly 50% better gpu (judgin by the diff between 7970 with 6970 and 7950 with 6950).
If you insist on taking one of two, then 6850 should be superior as an option.
As you can see here 6790 needs to be o/c so much that can reach 6850 which is 17% faster. So it should need to go over 1000Mhz to reach 6850 (and might not be able to reach its performance even then):

Well you should look for a 7850 or 7770 which arent far away according to MSI:

So from 15th feb to end of march we should have a lot new gpus from AMD and maybe a reply from Nvidia as well which this will make both companies to make better gpus and lower their prices.

As for MSI GeForce GTX 560 Ti Twin Frozr II is a very good option (around 2% better performance than 6950) but its highly priced. But if you find a good deal about it its very good. Actually everything depends on pricing.

Even if you want to buy a GTX 560 ti in my opinion you should wait a few days .