I've only had a grab of the Sony lens so cannot really say how it stacks up optics wise. The build is obviously better on the Sony, it is SSM v screw drive, it has sealing too and it's a bit bigger and a bit heavier, and slightly wider at 16mm. I did not have a chance to take test shots with one (shop play around)
Photozone have reviews for both as do others, going on that and other ones I'd say they are pretty close performance wise on optics with an edge to the Tamron at the 50mm mark (but copies vary on lenses)
Being fair to Sony the 16-50mm f2.8 is priced quite well (for once) v other brands who tend to have rather pricey lenses similar to this (with more pro like build etc)
I'd narrow it down to do you want/need the things the Sony offers and if not go for the Tamron. I can't really knock the Tamron the price is good and it's mostly a well liked lens bit of a bargain really.
Both are clearly going to be a decent step up v the kit lens
DK did a write up for the lens here this might helphttp://www.photoclubalpha.com/2012/02/1 ... -f2-8-ssm/