A very good question! And one I really struggled with for months before finally opting for the R3.
I can't speak for all the others but the biggest deciding factor for me was the ride quality. Since a (motor)bike accident a bunch of years ago, I have a few comfort issues on long rides. On test rides, (actual or not) I did perceive a difference in vertical compliance and comfort between the two bikes - I even ensured I was using the same wheels and tire pressure on the test rides.
If I was only racing on the bike, I would have selected the Soloist, hands down. But the reality is I'm going to do most of my training on that bike too. I'm not going to train on one bike and race on another.
Finally, I like the opportunity of using the R3's comfort to tackle rides that include some pretty rough terrain, dirt roads or even single track.
As for the decision to go with a Cervelo... aside from the fact that I'm Canadian and proud that a small Candian start-up company ended up truly producing some of the best damn bikes in the world, it's a sponsorship thing. So maybe I'll choose the Soloist next year - or more likely the SLC-SL. Unless Cervelo finally comes out with the R3 Cross.