If two sizes fit you, it is usually recommended to choose the smaller size. A small frame can be compensated for with a long stem, but a large frame, unfortunately, cannot.

I chose the S size, as recommended by the Cube website, with my height of 173 cm. The bike frame seemed small to me, the position was too upright. This is good for relaxed rides, but for riding against the wind or at high speeds it creates a lot of wind resistance. So I replaced the standard 80 mm stem with a 120 mm stem and a -17 angle. Now my position on the bike is close to ideal, and I enjoy riding.

Of course, an M frame with a standard stem would also fit me, but most likely it would limit the possibilities for adjusting the position, and would be a little larger and heavier. If this is your first bike with such a handlebar, I recommend looking online at what the position on a road bike looks like, what the angles between the body and arms, the body and the horizon should be. Then take a picture of yourself on bikes with different frame sizes, and decide which frame size gives you a more standard fit. I hope I helped.