Tourist in MSN

How are you getting to the start and end points for your tour? Would a trailer make logistics more complicated?

Uphill with a load with a low gear of 29 inches? I would not want to do that. My touring bikes (all use panniers, no trailer) range from 16.2 to 20.7 gear inches. My road bike lowest gear is 30.6 gear inches and my rando bike is 25.1 gear inches. I would not even want to carry a camping load of gear on my rando bike which has better low gears than your road bike.

I prefer to tour with fenders, looks like that is not on your priority list for either bike. But if you change your mind, the 520 probably is a better choice.

If you need a repair, I suspect any bike shop would have what you need for parts for the 520, maybe not for the high end carbon wonder bike.

The point made by Debade on riding position is quite real, do you want to be on your race bike in race posture all day long every day? If you are a former racer, maybe that works for you but not for me. I have a high angle stem on my road bike so it gives me a posture similar to touring bikes.

The 520 is older, how well maintained is it? Would that be more likely to have a breakdown?

I am guessing that the probability of a rack failure is similar to probability of a trailer failure, both are quite low but you should inspect both to see what you think.

Finally, which bike would handle better on a long day when you are tired and daydreaming and not watching the road carefully? Touring bikes are better at holding a straight line if you take your eyes off the road for a few seconds than road bikes.

So far you are focusing on weight, but there are a lot of other factors than weight on a bike for touring.