I have been lurking this board for some time but this thread had me register to reply.
Since you have been using both, do you think the difference in the ranges 50-150 vs 70-200 for crop (neglecting other differences) had practical effect in some way?
I am using a 60D with 10-22 / 17-55 / 60 macro / 18-200 IS / 50 1.8, and have been looking hard for a good stabilized 2.8 telezoom to complement the set for indoor sports, portraits, and general travel.
While the 70-200 2.8 IS is expensive, it hasn't been the price only which has held me away from that one - it has just as much been the zoom range. I have borrowed that lens from a friend but didn't totally like the long 70 mm starting point. I can imagine why 70-200 is a de facto standard short telezoom range: it's a great portrait range - on FF. It wasn't indoor sports season when I had that lens so I don't have experience of the range there. Otherwise, 50-150 seems to make much sense on a crop camera.
About the 50-150 OS not being FF compatible, for me that just sounds like a good thing. I have no plans to change to FF, and like the thought of having lenses designed for the type of camera I have and not carrying around unneccessary and unused glass volume.
The few reports I have found on the 50-150 OS indeed gives the impression of a premium grade lens. As much as I like the Canon brand and its lenses I already have, it looks like I have found the missing link I have been looking for in another brand. If Canon doesn't soon announce an EF-S 50-150 2.8 IS...