Si tu veut vraiment un des deux premiers que tu a cité , le sigma : sinon le 70/200 est bien , tout dépend de ce que tu souhaite faire comme photo.

Je te laisse le choix du traducteur , un test du sigma :

The Sigma AF 24-70mm f/2.8 EX DG macro showed an impressive performance throughout all characteristics. It is a very sharp lens at all mainstream aperture settings. Distortions, CAs as well as vignetting are moderate and usually nothing to worry about. The build quality is very fine and the AF is pretty fast despite the enormous glass elements and a conventional AF micro-motor. The biggest problem of the Sigma is not performance but its scope. When used on APS-C DSLRs the zoom range (38-112mm) doesn't really qualify it as a standard zoom in the true sense although it surely has some appeal for press and basic portrait photography. It will be interesting to see how this lens will perform within the full format test scope.