If you want the most basic CCM marker - buy the S6. It is, in theory, just a souped up sniper. Like a stock car is JUST a souped up sedan.
In my opinion, it is, out of the box, the best a sniper can be. If had to only own one - it would be a S6.
Buy a T2 if you want to be able to change the top tube. There is so little difference between the S6.5 and the T2 that this is the major one. I don't see a smoothness difference.
The S6.5 has the same bottom tube of the T2 as well as nearly the same bolt. The difference is milling and dual rods. I like the 6.5 over the T2 - but that is merely preference.