Wow this is a really hard question. One the one side, the 1080ti is much faster, especially a custom cooler 1080ti. On the other side you game at 1440p.
Intel gives you more fps at lower resolutions. The question is how many fps do you want. Since the Intel would give you (for example) 180 fps rather than 150. But at the same time, your overall fps at all resolutions, will be less, with the 1080.
And the GPU is easier to replace, in the future, while a CPU can last 7-10 years.
This has to be the hardest question I've ever heard honestly.
View: https://youtu.be/nvCF3OLZlzQ?t=563
Ok assuming you don't have a 200Hz 1440p monitor, the 1080ti + 2600x is the best combo overall. The issue is, and this is why I said it's complicated, Intel gets more fps in games at below 4K resolutions, BUT it requires a GPU that can get those fps in the first place.
Considering you want Overwatch and probably 144 fps, the 2600x is sufficient. I'd recommend you try to get a 2700x if you get the extra money, so you don't need to upgrade for many years (and be more future proof), but that's up to you. Then again if you have the extra money I'd recommend a 8700k. Again, the 1080ti + 2600x is perfectly fine for now, for your specific case (unless you want those 180 fps).