AZT, It would be an easy choice for me. I would go for the R5 since I prefer a lighter bike and more plush ride. For you, the S5 is the best choice due to your flatter terrain and faster speeds. The wild card is how the bike handles with a taller rider. BTW, BikeWNC is a taller and has test rode Cervelos. He may be able to help. I do not think there are any bad choices and it is a high class problem to have.

I rode the R3 but recent changes in geometry to the Cervelo line would leave me guessing how they ride now. If anything, the geometry is closer to the Tarmac I guess, but I haven't looked at that specifically. For racing in flatter terrain the S5 seems like the bike to have in the Cervelo lineup. I think though some loss of comfort might be felt over the R3/5 due to the bladed downtube. I know the Venge is less comfy than the Tarmac for that reason. Whether that matters is a personal choice. Cavendish didn't mind.

I am kind of surprised that efficiency is translated into more speed. I would think that the definition would be getting from point a to b using less watts.

The bike mfgs. are going all out to get your wallet open with more aero frame designs. I don't think it is bunk but unless you are a pro rider it will not make or break your riding. Full carbon bent is the most efficient bike I have ever seen but not going up a steep hill.

Faster using the same watts, or less watts at the same speed, boil down to the same thing, which is what I'm referring to as efficiency. Some times you are trying to minimize your effort while maintaining a certain speed (e.g., in the peloton), and sometimes you are trying to go as fast as you can without exceeding a power threshold (e.g., Off the Front, Off the Back, TT, etc).

Yesterday I saw a rider in one of those recumbent type bikes encapsulated in the aerodynamic shell. It was fast alright, very fast.

Maybe if they could design a bent to be both leg and arm assisted on the hills, it would work. Nothing beats being on all fours.