Originally Posted by ericm979

The LBS should definitely do a fit before you buy.

The differences between the RS and R3 are:
RS has 1cm longer chainstays
RS has curved seatstays
RS has a taller head tube
and the RS uses slightly cheaper carbon

The longer chainstays will make handling a bit slower and the bike more stable. That's not a bad thing as the R3 is fairly quick handling. It's a small difference that won't turn it into a truck. I don't know what difference the curved seat stays will make, as the R3's are pretty small as it is. The taller head tube means that you will need fewer spacers to get the bars high enough. If you prefer a severe seat to bar drop or have short legs for your height you may not be able to get the bars low enough. But that's much less likely than not being able to get the bars high enough on an R3. The difference in carbon won't be noticed.. it's like an extra 50g.
The RS is also cheaper, which is nice.

If you can't get fit on either one before you commit to buying, I'd go for the RS as it's more likely to fit. But buying a bike that expensive without test riding it is not a good idea- too many things can go wrong. I'd buy a different make/model that you can test ride and fit instead of a Cervelo that you can't test. A bike that doesn't fit isn't a good deal no matter how little you paid for it.