1T gives more performance and bandwidth. If you are running 4 sticks of ram though it is too unstable and you will have to throw it into 2T.
The difference between T1 and T2 is negligible meaning 1-2% difference in performance and that is mostly in memory bandwidth benchmarks. Real world you will see little or no difference in performance. Run what ever setting is stable.
You should use 1T wherever possible. It may not be noticably faster, but it is faster and every little helps. You can find comparison benchmarks in this article.
for those debating the benifits of 1T over 2T please read this to rest you soul. 1T vs. 2T . the comparison is in the 2nd post. there are other things you could learn from to settle some confusion in the whole thread.
