Hi Tony,
I am going through a similar scenario and have found a few things. I hired a Sigma 150-500 and found it was losing sharpness after about 400 mm. That said the newer versions seem to be much much better from the reports i have read. I follow a guy called Tony Northrup on YouTube and he did a side by side with the Canon, the Tamron and the Sigma. he agreed with the Sigma on the drop off on the 150-500.
That Said,
I played with the new 600 Contemporary model and found it a big improvement. Do note that there are 2 models of the 150-600, namely the contemporary and the sport model. The sport model is a little under twice the price of the contemporary model.
A lot of reviews are tipping the Sigma over the Tamron, the only exception being the weight if you are going out bush or up for a long walk (It comes in at about 2.6kgs for ther sports compared to 1.95kg for the contemporary I think ) It does have some real cool features such as customisation, via the USB dock, lens zoom creep button plus 3 auto focus modes including 2 lockout.
Have a look on this site here for some images taken with Sigma, Tamron and Canon lenses.
Big Zoom Review
A more recent review of the Tamron v the Contemporary.
Tamron v Contemporary
Some of the earlier Nikon also had a focus issue but this seems to have been fixed.
Interested to see what you think. I am leaning towards the Sigma.