Upload & Sell: Off
Can someone show me the image quality difference between the two ?
Or can someone try to sway me to purchase one ?
They are 2 very different lenses. I actually got the Sigma 70-200 OS, which is far better than the first generation Canon (its target when developed) and stands up to the II in places, but just places.
I hate to point you here- but this interactive chart is one of the better ones I've seen for lens comparisons - choose the lens and the camera and check the aperture and focal length. I'd say it's pretty spot on for my Sigma - which has some very complex characteristics. Here I've dialed in the fist and second gen Canons, unfortunately the II on a 5D mkII, the first on a 5D, but you'll get the general idea. There's also Bryan's charts, but these only tell part of the story. I think at least here the first gen compares admirably. The AF on the 2nd generation is really fast and accurate - although my time with the first one was a long time ago and my memory...
You don't say what you're shooting with (or what you're doing with the shots), but if it's FF you might consider the f4 IS. It too is blindingly sharp. Or, if you're really trying to stick to a budget and really want f2.8, do consider the Sigma for only $1250. Again, considering you've got FF. Otherwise, the 50-150 is a pretty good choice. I'm not entirely convinced I'd have gotten f2.8 once I got my 6D. The low light performance is so good f4 works just fine in most applications. I say this because sometimes hauling around a 3lb. lens keeps me from taking it sometimes - I call it a bag lens, because that's how it gets from place to place