I do not have the Tamron, so I can't compare directly. Generally lenses seem to perform best stopped down a couple stops, regardless of what their "wide open" point is. So my guess would be the 50mm would do better at f/2.8 than the zoom would.
If budget is a big issue you might consider the Sigma 24-60mm f/2.8. Cameta had them on Amazon for $200 and I really like mine, very sharp at f/4 and quick focusing. There is one on ebay right now under $200 (not mine and I don't know the seller).
If you are looking for a wide angle zoom it won't fit the bill but otherwise it makes an excellent normal zoom and has been one of my better price/performance buys.
Edit: Sorry, I did not notice the auction google took me to was ended. These were widely available for $200 until quite recently, if you are interested keep watching, one will turn up.