Toyo are discontinuing the T1-S in the majority of the sizes apart from the common ones ie 195/50/15, 225/45/17, 225/40/18 etc. The T1-R tread pattern is different from the T1-S. The center section is the same compound but you are right in saying that the T1-R now has a harder compound shoulder. I'll pm you a price for T1-R's all round if you want and if it competes with the current deal with T1-S's then I would recommend going for the T1-R's! They are probably doing a good deal on the T1-S's as they won't be around for those sizes for very long and Toyo are churning them out at silly money at the moment. As far as the Nankang's go, in my humble opinion, I would do anything I could to stay of them and get some decent rubber on your new rims.
Cheers,
Andy