For the Trek I would go for a 58cm and the Cervélo a 56cm. Both have relatively the same top tube effective length. The Cervélo is certainly more fun to ride, but I don’t know how it would feel after 50 plus miles. I’m someone who likes to go on long bike rides multiple times a week.
I am considering the Caledonia myself right now but availability is a bit of an issue. My next long ride will be 600km and then a 1200km after that. The geometry is nearly identical to my Cervelo S3 except longer chainstays and less headtube angle. So, a bit longer and a touch more trail for stability. Room for 32 mm tires and hardware for fenders. I do not know the layup of the carbon but assume very similar to me S3. It was designed for Paris Roubaix. What is comfortable for me might not be for you. I consider 50 miles to be a short ride. YMMV