To touch on the subject again, if it's an option, try them both out first and get the one that suits you best. Weigh cost vs performance, and see if the $1000 premium is worth it for you. If you are going to base your choice strictly on image quality, I'm almost certain you will not see a $1000 difference.
the overwhelming response is greatly appreciated.
follow up questions:
1) a youtube review of the 50L mentions that after some year, there's no more of this "focus shift", true?
*Focus shift is a design flaw, so the fact about 50L's made after a certain year is false. QC may have improved but the focus shift will always be there due to the design.
2) if the 50L is the only lens i bring, i'm naturally gonna be forced to shoot stopped down sometime, are the posts here saying that the 50L is not as sharp as the sigma at f/4 or f/5.6 as the sigma or other canon 50's? or that the 50L is "meant" to be used wide open? in contrast, is the sigma useable all across it's aperture range?
*I have no personal experience with the Sigma 50 so I can't comment on that, but stopped down I get great results from my 50L. There is more to sharpness than just the lens. If you have perfect lighting you can get sharp results from any of the 50mm options.
The statement; "The 50L is meant to be used wide open," to me means most people pay the premium to be able to shoot at 1.2, not necessarily that it needs to be used strictly at 1.2. To be honest, if there is enough ambient light I rarely shoot wide open unless it's the look I'm trying to achieve. But... when you need 1.2 you need 1.2, and having that option is priceless. Does the 1/3 stop justify the $1000 premium? For me no, but that premium does give you partial weather sealing, great build quality, great optics, and the red ring "prestige." I say great instead of better because I haven't physically held or used the Sigmalux.
3) i've tried the sigma in the store, but haven't tried the 50L yet. although i could, i doubt a day's playing with the lens will push me either way.
*When I downloaded my first photos taken with the 50L I didn't regret not choosing the cheaper alternative, Sigma.