As far as comfort, both have the similar seat stays. I brought the R3 home for a two day trial over the weekend. That way, I was able to compare my RS to the R3.
I store my RS in the garage hung up on the wall by two brackets holding up the frame. The R3 would barely fit. I say this to confirm the geometry comparison of the two bikes. The RS is more stretched out with the longer chain stays. This doesn't mean that its some kind of touring bike. The RS still moves fast. But with the longer wheelbase, its probably more stable on descents. But if you examine the Cervelo geometry stats, the RS has an extra column for "rake". This tells me that the bike still handles exceedingly well, even though the head tube is longer.
To me the difference between the two is about acceleration and handling, two things the pro riders demand. It was most noticeable during ascents like going through a switchback and off the saddle for a surge on the straightaway afterwards. On flats, not so much tangible difference.
For me the bottom line was it confirmed that the RS I have is a good bike. Good enough that I installed the Rotor chain rings and PSIMET carbon clinchers.