I was swooning for that Sports lens till I saw the price.
I'm in the same boat. It is sharper than the Tamorn at 600mm wide open, but not night and day. Similar with focus ability. The other thing is it is twice the weight.
However, Sigma upon seeing the Tamron copy deiced they need to have a similar lens so there is also a Sigma 150-600mm f/6.3 C, C for contemporary, vs S for Sports. Confusing.
The C version from the manufacturers MTF charts looks like it could be sharper than the Tamron version. It is slightly smaller, lighter and cheaper than the Tamron. However, there are no real review or tests of it yet, should be some soon.
Depending on when you want the lens it might be worth waiting to see how the Sigma C compares to the Tamron (or Sigma S).