The S designates a longer electrode, I was told that they last longer.
I also heard that the tip is farther into the combustion chamber, and on high performance engines with large piston domes, the dome can strike the plug. Don't know if that's true or not.

 

Depends on hp and compression and other variables. 45 is hotter 44 colder ect. I'd say if it's mildly warmed over a 44 heat range would suffice. Take one out of the engine and see how far the discoloration goes down the ground strap. If it's about 2/3 the way down it your close on heat range. Porcillan color will tell how close on fuel mixture you are.

 

Timing will also have an effect on the ground strap, moving the discoloration and changing the colors, so timing should be played with before jumping to changing the plug heat range. I argue that if it's a mild, mainly street-driven car, the 45 heat range would probably be better at frying deposits. Then again, I guess the OP might know what heat range the engine likes.

 

The piston can certainly strike the spark plug, but it depends on the plug, the piston, any decking/milling, etc.

If you're not running aggressively domed pistons, I'd go with the R4#TS/R4#S depending on the seat design.