The 100-400mm will be sharper than the 150-500mm , IMHO. I had one it was super sharp on the 400mm end.
The 100-400mm: Is very very sharp at the 400mm end if you get a good copy--almost as good as the 400mm 5.6 prime--or honestly, as good, in the center anyway. Super good. Not so great at 100mm wide open though, that's for sure--lacks contrast...stop down. It's also extremely light for what it is. However, the tripod foot is TINY, you can't get your fingers around it unless you have tiny tiny hands, so it's a pain to carry imho--put in bag, on tripod, or use a black rapid style strap. The AF is good, great with the limiter. The IS is kinda crappy. =p Examples: tons on my site and tons in the thread here.
The 50-500mm OS. The copy I had was *excellent* in the center at 500mm f/8. I was using on a D800. The OS is quite superior to the Canon's IS, imho. The AF is similar, maybe a tad slower and less accurate...The range is nice, and the macro capability is great. I had to return mine because it had electrical gremlins. It's maybe not QUITE as sharp on the long end as the Canon, but you have a little more reach to play with--it's a wash. It seemed to have a lot more CA and purple fringing than the 100-400mm. The CA can be removed in post, though. It's astonishingly good from about 135mm to about 250mm, heh---f/8 corner to corner sharpness on D800---I was...surprised...to say the least. 50mm a bit softer on extreme corners with a lot of CA. 500mm corners are very soft.
Some have reported low contrast from this lens, but mine was sharp and contrasty...examples from the brief time i had the 50-500mm...this was on a full frame cam:
Does okay at longer distances:
Extremely heavy crop, here...sharp and contrasty, huh?
Has no issues pulling feather detail out!
Downsides of the 50-500mm OS: Might take more tries to get a good copy (I was sad when I had to return mine, it caused the camera to display f/90 and lock up!) It's quite a bit heavier than the 100-400mm. However, it's also easier to carry because the tripod foot makes a great handle, I found it easier to carry than the Canon.
120-300mm OS: First copy I had didn't auto focus. Traded for another, it seems great. Seems to be in another league from the 100-400 and 50-500...it seems more similar to a 300mm 2.8 prime honestly. It's MUCH larger and heavier than the other two lenses. Fortunately the weight isn't out away from you like it is on say, a 600mm f4, so it seems quite easy to hand hold and carry. IQ is very, very, good. Haven't tried with a TC yet. The OS is good, I'd believe the 4 stop claim. AF is good, seems to be very accurate (this is on a Nikon though I dunno if that changes much), speed wise it's about the same as the 100-400mm. Maybe a little faster--but a little slower than if you had used the limiter? Meh. Not as fast as, say, the Canon 300mm f2.8 IS prime. At 300mm it lets in more than four times as much light as the 50-500mm. Corners seem incredibly good stopped down a little. Think of this as more of a 70-200mm f2.8 IS mark II that zooms to 300mm. I'd say do it if you can, and can stomach the weight. If not, decide if you want light weight, worry free sharpness, or larger range, better OS, useful macro mode, but maybe a bit more trouble to get a good copy with the Sigma. (Please, I'm not bashing Sigma here, I love them and love their stuff, just going off of my experience).