First of all, depending on usage, the SL6 could have less wear, but that is highly dependent on the exact bike you’re buying. There’s wear you can see (drivetrain) and wear/damage you can’t (frame internals, fatigue, bearings). So talk to both owners too at least understand how many miles / hours the bike had been ridden. Ask about any crashes too.
I would lean towards the SL6 for a few reasons.
- Thru axles means the bike has more resale appeal if/when you move in to something else. It also means investing in wheels for it can likely transfer to new bikes / other bikes.
- Supposedly the SL6 is more aero and light as a frame itself. This is the one part of the bike that you can’t change, so having it be better is a positive.
- The SL6 was designed when wider tires were being acknowledged as faster. It’s got a bigger tire clearance (officially a 30 mm tire vs 28 for SL5).
The main downsides to the SL6 are the 105 instead of Ultegra and no carbon wheels. Depending on wear and your upgrade plans, you might be swapping out parts anyways so that’s not much of a factor. Besides 105 is still great. For the wheels, that’s personal, but the cost of carbon wheels has been coming down a lot.