I own the Tamron, and it's a great lens. That said, if I were doing it all over I'd buy the Sigma Contemporary in a heartbeat. Besides having a slightly nicer build (and looking better IMHO), the USB dock is a huge advantage. You can fine tune your lens without resorting to in-body micro adjustment. And, you can update your firmware, which is huge.

I just sent my Tamron in for the service advisory - there's a problem when panning the camera with the VC on. It cost me $50 to ship the lens off. If it was a Sigma, I'd be done already. That $50 would pay for the USB dock. And, I wouldn't be without the lens for 2 or 3 weeks. While Sigma claims a 3 day turn, here on the West coast it's a week to get to NY via UPS ground. This is the second major firmware update for this lens. I doubt it will be the last. I'm hoping that I get some performance increases as some users have noted after this update.

Then to answer your question - as busted knuckles mentioned the weight is a huge issue. Holding a 7 lb. lens is tough even just for a few seconds. I've found I need to have my shutter speed at at least 1/1000 to get good results. At 600 I can see my image bouncing around and know I need that shutter speed and VC. I bought a monopod pretty much exclusively for this lens. I've gone 40 years without one. It makes a tremendous difference. It's also a less expensive setup than a tripod and Gimbal would be. I'd like to try a gimbal at some point in the future. A good Gimbal is an investment by itself, but I think if I get good with birds and move to BIF, it will be an essential item.

So anyway, the Tamron is a great lens, but unless you're a huge Tamron fan, consider the Sigma. I believe Bryan gave a slight advantage to the Sigma himself. If anything happens to my lens during shipment, I'm using that insurance money for a Sigma!