I'm no pro but I can tell you what I like and why.
I have had a '05 and '06 Roubaix Pro. Still have the '06 and a R3 now. I ride all kinds of stuff, lots of old rail road beds that most use Mtn bikes on. My tires are 23mm Pro IIs for fun and have some 25s and 28s for the same courses. One is up to 21 miles all down hill at 2 or 3%. R3 is just a tiny bit faster, a bit hairy/squirely because it is so stiff and short. Roubaix is more comfortable and almost as fast but always there and not squirely at 25+mph on skinny tires in loose gravel.
I also like to climb some pretty steep stuff...up into the low 30s on pavement, which is mtn bike steep for me.
The Roubaixs are slightly heavier except for the new '07 which has to come close to the R3 since my '06 is just a 1/2 off fully kitted out. But it is enough I noticed because of the balance that you really feel it going up hill when you stand up. The R3 will scoot out from under you. The Roubaix gets the job done but doesn't feel featherly light doing so. Not a bad thing but the R3 does. That said the geometry on the Roubaix is a lot easier for me to be comfortable on and I climb on it much faster. Bikes are kitted out as close as I can to being exactly the same. Had to change to a longer stem on the R3, move the seat up and still doesn't climb as well. But it is better. Just not the power for me of the Roubaix's position.
Love the Roubaix and if I could only have one, it is the bike. (preferably a new SL ) But the R3? Oh, my the frame is solid. Never felt so connected going down hill. Feels like a good super cross motor cycle on the down hill, it is that solid. Doesn't feel like a bicycle. Amazing a lwt climbing bike feels like this going down hill. Solid and fast...and secure. Light years
better than the Roubaix on a steep paved decent. BB stiffness? The R3 is everything they say it is. Unbelievably stiff. You feel it in the first pedal strokes. No kidding. I knew the difference in two flat blocks in front of my house. It is that much stiffer than the Pro, which is a fairly stiff bike.
Comfort? Don't let Cervelo fool you. It is good but the Roubaix kicks the R3's arse on that issue. Done a couple of hard dbl centuries in a day on the Pro....not in a hurry to do one on the R3. But I could also see the R3 being faster if I could take the extra pain. (nothing as bad as my E5 Sworks for comfort although the frame specs are close I bet on the R3)
I haven't gotten rid of my R3 and may not. It climbs fine up to around 20% Not as fast but then I feel like I am much faster and more secure
on the downhill side which is a good enough reason to keep it.
As everyone has said both are great bikes...but my hat is off to Specialized, the Roubaix is a great frame. When Boonen wins a bunch of classics on his Roubaiz this spring we'll all hear just how good it really is
To sum it up I wish I had not sold my '05 Pro. It fit me better than the R3 does. I am looking for a S works Roubaix (for the extra stiffness in frame and BB) to replace it after tasting the obvious pleasures of the R3.
I was hoping the R3 would be my last bike for some time. But I'd like the best of both worlds.
Forgot..R3 is $2800 for the frame. The Sworks is $3500. Nice frame but $700 more is really pushing it.