18-50 cmi at wide open f2.8, but infact, so are all the 3rd parties... (for F mount that is)
Having used them for work and still having the 18-50, AFS17-35/28-70 with me now..
Fortunately at least, the sigma gives very sharp shots comparable to the trinities/ or AFS17-55 at F4 and above. If you are looking to use for events and will be using with flash/bounce/LS most of the time, then there should be no diff from 17-55...