[danpen]

Chris,

Both of these lens are for the cropped sensor only - from the Sigma Site:

DC (DC Lens)
These are special lenses designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras. Their specialized design gives these lenses the ideal properties for digital cameras, the compact and lightweight construction is an added bonus! including compact and lightweight construction.

I've never used either of these lenses nor have I ever used a Sigma OS Lens so take this with a pinch of salt. Generally a lens with an 11x zoom range is unlikely to be excellent at any point and quality may drop significantly at one end of the range or another, have a think about whether you really need all of that in one lens.

Looking at your kit list, you've got all the range covered now so there's no point buying one of these as a "stop gap" it's unlikely to give better quality than you have now. Unless the convenience of a single lens is really important then I would say have a think about the range you use most and get some better quality in that area. That Tamron 28-70 gets good reviews, the 50mm will be great. If it were me I'd be tempted to either replace the kit lens with a Sigma 18-50mm f2.0 EX or replace the 70-300 with the Canon EF 70-300mm f/4-5.6 IS USM.

That would give you good coverage with three not too big, not too heavy, pretty good lenses.

Cheers

Dan