Core clock is the speed at which the gpu performs its tasks, overclocking gpu core usually increases fps and overall performance of a card. How ever you dont only look at the core clocks to see which card is better since its the architecture of the card which matters. Its like buying a pentium 4 3.06ghz over a 2.26 ghz I7 cause it has a higher clock. It doesnt work that way, you need to review both cards, check benchmarks etc to see which card is better.
I use this site to see which card performs better ----->>http://www.videocardbenchmark.net/high_end_gpus.html
According to the site the 6790 out performes the 550 ti so that would be an ideal choice.