S.Horton wrote in post #14011243
So, what is it you think you will get that your 17-55 isn't already giving you?
How often do people really need photos taken with their ultra-fast lens wide open? I'm thinking that it's not as often as some would have you believe.
And how often does the extra "depth of field" of the 5D MarkII come into play?
Faster lenses do have a distinct advantage in that the wide aperture makes a significant difference in low-light autofocus ability. I think that's the primary advantage to using fast primes, unless you're really into photos where the eyes are in focus and the nose is out of focus. And, the 5D II does have larger receptors, which evidently does make a small difference in dynamic range and noise reduction. (Something about averaging out the photons and not being corrupted by singular small anomalies striking a smaller receptor.)
But whenever I hear about the incredible differences between the full frame and crop sensors, I think of the guy from the sixties who offered a $1,000,000 prize to anyone who could prove a paranormal claim under scientifically controlled conditions. Everyone talks about these huge differences between sensors but I've never seen two side-by-side shots where someone says, "This is the obvious difference between a crop sensor and a full-frame sensor."
The reason full-frame-taken images look so good is related to the Brenizer method, where multiple images, taken with a shallow depth of field, are stitched together to create that desirable combination of shallow DOF and wide Field of View. (I think that a full frame's wider FOV makes a small, but noticeable, difference but that the greater dynamic range and noise reduction advantage is greatly overstated.)
Sorry to ramble, but I think the most significant gain (if you really want to get a prime lens) would be with something like an 85mm f/1.x that could be used for shooting sports with super-quick autofocus and a fast shutter speed. Your shots already look good - an 85mm prime would be a nice addition for shooting from a short distance.