I’m 5’7 and have just bought an Aethin in 51 as a replacement for a snapped frame. Haven’t even built it up but I can tell you it’s shiny.
Hope that helps…
To give a bit more help, I based the decision on the geometry of my summer bike; a Felt F1 which I’ve ridden for the last couple of years and suits me perfectly
The Aethin is 5mm longer in the top tube than the Felt and I’m putting on a 120 stem, as I run on the Felt, but with a 5mm shorter reach bar. I prefer a smaller frame/longer stem fit for the frame stiffness and handling sharpness.
That said, the Aethin also has a longer wheelbase for stability which should be good as this will be my winter/bad weather road bike.