Hey Pheo, Ive never been to Venice but ill play :P.
The biggest decision for me would be between; the Tamron 17-50 or Sigma 8-16. (My personal kit would consist of 31 ltd, 16-50 and 50-135 in Crumpler 5MDH)
Sigma 30 1.4 would be amazing for low light, museums, portraits of you/girlfriend in front of stuff etc.. Personally as a rule I never leave the house without my da 50-135 but mine is being repaired atm
! The Da * 50-135 however, will be great for detail shots, isolating interesting people etc.. you just have to learn to love its size. Sometimes I leave the lens hood off as it reduces its presence on the street.
I think the 8-16mm would be awesome to gain a different perspective. I have been eyeing the lens as my next purchase since viewing heie's (PF member) shots of NZ.
Good Luck, Would love to see the pics when you return
D.Vaughan