I would say there will not be too much of a performance increase you are going to get in ESO with the CPU upgrade. ESO uses the HeroEngine and the only other game that uses that same game engine is Star Wars: The Old Republic. Below is a link to the only CPU benchmark I have found that basically compares 2nd gen Sandy Bridge Core i3 / i5 CPUs to AMD's Phenom II. The Core i5-2500k is a without a doubt the more powerful CPU compared to the Phenom II X6, but the i5-2500k only gets an extra 5 FPS in the benchmark.
http://www.tomshardware.com/reviews/star-wars-gaming-tests-review,3087-8.html
GW2 uses the Unreal 3 game engine which has been around for a long time and performance should not be affected too much by the CPU unless the CPU is so slow that it bottlenecks the GPU. So I don't think you will get a big increase in performance by simply upgrading the CPU. Games using the Unreal 3 engines are generally easy enough for most people with a good (not great) GPU.
The exception is Iron Galaxy's crap PC port of Batman: Arkham Knight developed by Rocksteady. It was so bad Warner Bros. pulled it from Steam and other stores soon after it was released back in June. I think it will be re-released near the end of this month (unless it has already been re-released). People say the patch for Win 7 and Win 8 makes the game run much better, but they are still working on a patch for Win 10.
If ESO and GW2 are the only games you are currently playing and you are satisfied with the performance you are currently getting, then I would stick with the Core i5 750 for now. However, if you feel the itch to upgrade, then go right ahead since other games that you decide to play can likely benefit from better CPU performance. However, do not neglect the GPU because the biggest gain in game performance typically comes from a more powerful GPU.