I think im going to go with the Sigma.
I read this great review on http://www.lenstip.com
The image resolution for the Sigma blew away the Tamron.
They had these nifty charts for the testing also.
"It’s worth reminding here that the best „primes”, tested on the 50D, reach the level about 52 lpmm and results near 34-35 lpmm we consider to be the decency level."
So the Tamron is a really good lens based upon the average range for most lenses but the Sigma is significantly higher quality.