I’m wondering how much the shape of the nozzle tip influences the quality of the curved undersides. I might be wrong but could it be that the UP is better because it has the flat nozzle tip? I imagine it like this: the flat tip may create more even “plateaus” which gives the next “offset” layer line a more consistent edge to sit on. The Zortrax has the rounded tip (…in my case it was even asymmetrical) and I could imagine that the plateaus are less consistent, allowing parts of the next layer line to “drop off the edge”. At least on the raft you can see a huge difference in the way the lines look between the 2 nozzle types. Again… I did not do any testing yet and I might be totally off here. It’s just an idea! 
here is an exemple of the print I did yesterday, where you can see the problem on the top :
Not only it was a nightmare to remove the support, but I can’t even use it, as there are holes on top of the head… ;(
and here is another print a few days ago. After painting in red.
