I am about the same height as you (5' 11"). I think we roughly straddle the 2 sizes. I purchased a 54 Roubaix in 2013 and have ridden it many miles. Yesterday, I took delivery of a 56 Aethos. I'm still dialing in the fit, but it feels good so far. I only have about 10 miles on it so far. Plan to stretch it legs this weekend.
Interesting you ride a 54 Roubaix. I would have thought a 56 would be more appropriate for your height, especially considering its endurance oriented geometry. Do you use a longer than standard stem to ensure enough reach??
Your 56 Aethos has about the same amount of stack as your 54 Roubaix (565mm vs 563mm) but a lot more reach (395mm vs 380mm). At 392mm, even my 58 Roubaix (2020 model) (for reference I'm 6' and have a 36" cycling inseam) has a shorter reach than your 56 Aethos.