sal 85 2.8

Hi all, hope you can help.

I would love a sharp portrait lens for my a65 and understand the sal85 2.8 would meet that and my budget.

However I already own the Tamron 70-300 usd, which will do f4 at 80 mm and is also very sharp.

My question is, is the sal85 2.8 worth buying, other than an extra stop what would the lens offer me over the Tammy?

Thanks:-)