The R3 is very reactive to rider's input. you can sit on a group ride right in the middle of the group and the moment you step on the pedals the bike just jumps forward. also, when you ride the R3, you feel you are integrated with the bike as one unit. with the Paris, it feels like you are in a double decker bus, you feel on top of the bike. you can ride the R3 for 50 or 100 miles and it will not beat you up.
now, i woudl not pay the extra money for the SLC-SL. i think the SLC is the perfect the way it is and i would spend the extra money on a nice pair of wheels. one thing is that both the SLC and SLC-Sl will not be as forgiving as the R3 after 60 miles or so. at least in my experience.
I like the R3 in white. is the R3-SL worth the extra dough? not on my book. i will just spend the extra $ in wheels like i said above.
But, now, there is the Isaac Impulse. i rode one last saturday and it felt just like the R3, but, i felt that when climbing, it responded a litttle quicker than the R3. Descending is where the Issac takes the R3. the Isaac has a 1 1/4 bottom head tube and a higher head tube also (174mm for the 55.5). these 2 combined made the isaac more secure when coming down hills.
remember, we are comparing top of the line frames here. the fact that the isaac descents better does not make the R3 a bad frame choice.