2x TC for 70-200 2.8 IS vs 100-400L

I recently picked up a 70-200mm 2.8 IS which I'm quite impressed with. However, I'm interested in a longer zoom and am currently wondering if it makes more sense to pick up the 100-400L or if I'd be better off picking up a 2x TC for the 70-200. On one hand, I think it'd be nice to have the 100-400 range in a lens without using a TC as I may get better IQ, but it seems that with the 2.8 I'm getting the benefit of a wider aperture over effectively the same zoom range.

Thoughts?