I have a Cube 56cm and Im 5'9"
If you look at the cube geometries, even though the bike is a 56, there is nothing measuring 56!
on the Agree, the 56 has a 54.5cm real top tube and a 52cm seat tube, so it would be comparable to other brand's 54 or 52 size... Sizing is not transferable from brand to brand, you have to look at the geometry
In my case, I think the headtube is a bit too high for me, and the top tube a bit long, so I am using a shorter and steeper stem... But this is also because I like a quite aggressive position, even though I am not that flexible....
Bottom line, it all depends on how you are, flexible or not, build, position you want to ride, but if your LBS(or even a not-so-local) has both sizes, your problems are solved...
There's a 2011 version of the cube in the classified section of this site ate the moment. It's a 56 and going for £850
Works out at £1132 quid. (great to buy from by the way)
Hang on just realised thats not the pro they have in red and white though?
Hope this helps with picking the right size, I have an Agree GTC Pro (2011 - Red/White) I'm 5.10/11 and went for a size 58 after hearing about the sizing/geometry and it fits just right for me. I've done long sportives & Alpine climbs on it without any probs, it's a pleasure to ride.
I previously had a Giant in a 56 which also fits Ok.
Patch, Graham here, sorry they won't let me reply to you. I sold the bike on here in the end. Hope it helps