As an S4RS owner and a Sport Classic owner I hope I can shed some thoughts on this.
The 1100DS motor is a fantastic machine, the pinnacle of the air cooled engines. It's a little more oomph than the 1000 but not much. Now the S4R will have the 996, not the 998 my S4RS has. These are not 100% apples to apples comparisons to the S2R or S4R but not everyone will have damn near the exact bikes you are comparing in their garage, which I do.
I enjoy the riding experience more on the Sport Classic than the S4RS. The 1100DS makes very nice power all the way across the board. There is a kick in the pants around 4000 rpm as to be expected. With some clutch slip it will easily tip the front wheel to the sky. The S4RS is completely different. It's all torque all the time. It wants to pull the front wheel all the way over just because. No clutch needed, no suspension bounce, just twist the throttle. The rear tire breaks loose for any reason whether you ask for it to or not. If you can master sliding the rear end out while roasting the back tire through a corner, oh you are in for a treat. There is a reason mine is set up for monoposta only. I don't want to involve anyone in the madness.
If you are commuting or just riding around and want another machine, the S2R is a better bike. If you are looking to add some madness and mayhem the S4R kicks it up a notch. Not quite as big a notch as the S4RS but it will be a different bike than your S2R. It will also handle much more different as there is a weight forward bias with the radiator and larger engine parts.
If you are looking for an interesting air cooled replacement and just want to try a different machine, maybe check out the GT1000.
09 Aprilia RS125 plated< NEW info> Now a 144 and back on the road!
06 Paul Smart Sport Classic NFS ever.
08 S4RS Tricolore #081 NFS ever.
The pink Monster has moved away.
97 900SS CR
72 Norton Commando 750
03 KX60 with S4R rear shock,Honda CB350F, Hodaka Ace90, 3x Yam-YGS1, Yam-CS3 200, Vespa small frame, Gilera 106, Puch Sabre, Puch 50 Boy Racer, Benelli 250, Benelli 360, Honda CB350T, and many more.