i'm trying to take a survey with regards to performance of several lenses of choice. the expensive 24-70 by canon or the sigma's version or maybe the tamron 28-75 . . .is the sigma 17-70 also ok? how about its 18-50mm? im really undecided so maybe you experienced guys could help me out. budget is about 1k . . .heheheh . .dont want to spend on things i cannot maximise the use . . thanks!