You need to keep the 70-200. The 150-600 is way too big and heavy for general use at the wide end - it's a lens that is only meaningful for people who do need lots of reach.
Yeah, that is also my thoughts. Well, and I guess that's my worry about the 100-400. I rented the 70-200 2.8 IS II before, and it was pretty damn hefty, compared to my 70-200 f/4.
Which is also basically the same weight as the 100-400.. BUT, i was holding it in my hands for 2+ hours straight. Now I have a black rapid type sling/strap, and it would make it much much easier.
But yeah, as you said, I wouldn't want/need to carry it around all the time. Actually, I usually end up using my 135 F/2 most of the time, and figure out compositions, or just use my iphone as secondary for now.
I found over the last year, the vast majority of shots I take are either 11-18MM, a chunk between 70-200, and most of those being at 70 or at 200. I rented the 300 F/4 + 1.4x extender, and took alot with that setup as well.