I vote for the 2.3. I don't know how it shapes up to the C'dale on beefiness, but Trek tends to have a slightly better feel (at least for me) on their road bikes. Everybody is different, and a lot depends on how you fit with a particular bike, however. I do know that the Trek will be solid, and will last you forever, I just don't know how it compares when you really go down on the cranks, but it should be a close comparison between them.
The extra $400 dollars for the 5.2 will be worth it, but don't make the jump unless you can really, really afford it. It's still not likely, but it is much more possible to mess up a carbon frame beyond repair than it is a aluminum frame. I'd not buy it unless you can afford to lose it if the worst should happen.