The 70-200 f2.8L IS is a *fantastic* lens. IMO it's one of the best zooms that Canon makes. It is very sharp, and it has an excellent bokeh.
However, as others have pointed out in reviews, the 2x TC isn't that good. It decreases light by a LOT (2 stops), and it decreases sharpness. I have a 1.4x and 2x TC, having purchased the 2x for the Antarctica trip because I wanted more reach -- and shots I took with the 2x revealed it's not so great.
So it's a mixed bag... I think you have to prioritize. If you want to favor the shorter focal lengths and get the best overall lens and occasionally go longer, go for the 70-200 with the TC. If you spend more time shooting the long end (perhaps you like birds) then go with the 100-400. If you can give up a bit of reach, the 70-200 with the 1.4 TC may be a decent compromise. The 1.4x is substantially sharper than the 2x -- it is entirely usable and auto-focus doesn't slow as much as it does with the 2x.
I could really care less about birds so personally I would strongly favor the 70-200. If I really wanted to go out and shoot wildlife or whatever I would probably rent a 500 f4 for the occasion... given I could probably get it for $50 for a weekend from the local shop.