I was going to ask you if the Yamaha required a MC license... but you already know.
I would think the Yamaha would be easier to sell (in the US) because everybody has heard of Yamaha, and there are Yamaha dealers all over the place, I don't even know of one Derbi dealer.
Plus that Yam Zuma has a reputation as a very high performance 50.
If it gets the same reputation as the Yam YSR50 it will become a "cult bike" which are worth what they were bought for 17 years ago... you can't say that for many bikes.