I don't own either, either, but I was very temped by the Tamron before I went and blew my budget on other things
. My opinion is purely theoretical, but I would go with the tamron for two reasons.
1) It is faster, and I find the difference between f4 and f2.8 very large (I can get away with 2.8 indoors with ok light, but f4 is flash territory only).
2) f2.8 + 50mm reach = reasonable portraits. You might lose a few degrees on the wide end (which you might, or might not, miss), but you gain more on the long end and you get the nicer bokeh. I'd imagine you can take much nicer portraits with the Tamron than the 16-45.
Therefore you get a wide angle, normal, and portrait lens all in one, which I believe would justify the cost difference between it and the 16-45 which would essentially be a wide angle and normal lens.
Wait for the people who own it to be sure, though
.