I ride regularly for exercise in New York City but do not race. I currently ride on a steel frame that handles the rough roads easily. I want to buy a carbon bike and am torn between a "classics" bike like a Domane or a Roubaix and a race bike. My rides are usually around 2-3 hours so comfort is nice but not essential. I appreciate quickness in traffic and am comfortable with a race bike geometry but am worried about durability. Any advice on how to think about these trade offs?
I think you answered your own question when you stated "I don't race." I know many in our club who enjoy the Roubaix. I personally prefer riding my steel Soma ES over my carbon race bike for most things, including fast club rides. Don't get me wrong, a twitchy race bike is great in a crit, but the other non-race bikes have had their design optimized for non-race road riding. It is too easy to get up caught up in all the bike porn the industry pushes. A solid, well sorted, well maintained, properly fitted road bike of any class and age can be a great riding tool.
In the end this is just all opinion, but I would be careful not to get caught up in the industry's push for "race" performance, which only serves a small minority of road riders.
Ask yourself a question...if you end up getting a Roubaix, for example, will you buy it and still be hankering after a carbon frame? If that's the case then perhaps it would be better to get the carbon frame now rather than to get a bike, be not entirely satisfied with it, and end up buying a carbon bike anyway in a year or so.
I just mention this perspective because that's exactly what happened to me. Nothing to do with what you actually need, nothing to do with whether you'll race or not, in fact quite irrational. But that's how many people think (and of course the manufacturers know this - those $10,000 bikes sitting at the top of their range aren't aimed at the professionals, but at hobbyists with money to spare).
The only concrete thing I can add is that I previously owned a 2010 Trek Alpha 2.3 - same geometry as the Madone but an alu frame. Obviously we're all different shapes and sizes but I found it easily comfortable enough for long, six hour rides, so I doubt you'd have a problem comfort-wise. And durability? Well, look at the warranty periods the manufacturers offer on their frames and that should give you an idea of what they think the odds are of failure.
Hope this helps.