I have the 7D.
I would only buy full frame lenses. I bought many of mine back in film days (full frame), and one day will go back to full frame in digital.
I have the 17-40 F/4 L and 8-14 F/4 L fisheye as my wide angles.
I think you need to factor in what you're wanting the wide angle for. If it's to get more sky in one frame, then yes the sky is at infinite distance and the only way you'll fit more in is to get a wider angle lens. If it is to fit more landscape in, or to achieve a different effect with landscape, then it comes down to technique (including positioning yourself relative to foreground and distant objects) as much as the wide angle. It can be surprising how wide field a photograph can look even though it's taken with a 24 or 35mm lens. I learned the latter thought John Shaw books I bought (back in film days).
On the 7D... The 17-40 is very wide and will fit the central bulge of the milky way no problem. Obviously the 8-14 will fit a lot more, basically a whole half the sky pretty much, which is huge. You get the fisheye effect with the 8-14 which suits some shots and not others. Both lenses are top quality, especially the 8-14 which is extremely sharp and pinpoint undistorted stars to the edge at F/4.
Btw, great shot
... which is why I came to the thread in the first place
