If you are ok with just 30fps then the 980ti or tix will both do the job, the tix wins a little bit in some games but just barely and not really enough to justify the premium over the 980ti.
But to really use 4k and approach 60fps you need a dual card solution, and now a pair of 980s can be had for less than a tix. Dual 980ti cards would be even better with the extra vram. Although the 980's 4gb of ram is low for 4k, I hear dx12 may fix this soon.
Sure you can fudge the settings to get better performance out of a game, but what's the point of 4k to only run at lower settings?
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