I had the Tamron 150-600mm on order for several months. Then I cancelled it and went with the Canon 400mm f/5.6 instead. Subsequently I read about the issue with the Tamron in VC mode while panning at low shutter speed. That sealed the decision for me because I am also interested in aviation photography and need to be able to pan at slow speed for propeller blur.
I had to rent a 100-400 this weekend for the Duxford Airshow, the firm I hire from has Tamron 150-600's so won't go for either Sigma. I had the Tamron from them for a show last year, but suffered so badly from the VC issue that I would have got better results from my old Sigma 26-300. I'm going with the MK I 100-400 as the difference in hire cost is fairly significant on my fixed disability benefit. The only good thing is, the lens arrived today for a three day hire (Thurs/Fri/Sat), and thanks to the holiday weekend here I don't have to ship it back until Tuesday.
I really wish I could visit some of the National Parks in the US, and of course Alaska. Unfortunately though my hiking days are over, as I need wheels to get about these days, although it makes carrying the camera gear a little easier.