Having owned both the sigma 70-200 OS and the sigma 50-150 OS I can say that the 50-150 OS had better IQ wide open and, depending on what you shoot, is more versital on a crop camera. If you want a 70-200 then I'd suggest the tamron VC over the sigma OS
The going ff options is something I struggled with as well. FF is constantly getting less expensive but what offset it for me was the fact that sigma had two amazing crop lenses (18-35 and 50-150) so that cemented me in the crop category for the forseeable future

But that's just me. Go over what ff will offer you over a crop body and if te cost difference (both in bodies and lenses) is worth it to you.
I've toyed with the idea of the 17-50 and the 50-150. The only things full frame really offers is a better choice of lenses (variety) without having to worry about crop factor (ie., shooting room), and image quality (low light, dof, noise in general). I'm sure there are a few bells and whistles that I'm missing. But, as far as the variety of lenses, how many do we really need, right? I'm probably going with the 70-200, though, for the possibility that I may do full-frame in the future and for retaining value in the lens. I'd love to hear thoughts on this, however.
As far as Tamron vs Sigma... yeah, if I do the 70-200, Tamron was my first choice between the two. But there's a $300 difference in price. I'm not sure if the IQ is really worth that difference. It may be.





