We are also looking to switch to a "composting" toilet, haven't really decided between the NH, Airhead and Separett (Villa and Tiny), but leaning towards the Airhead. Must say I'm surprised with your experience with the Villa, in particular your bad experience with the the urine collecting basin. Now I have never in person seen/touched any one of these so called "composting" toilets, but from what I've seen (I could be wrong!) the Separett Villa and Tiny actually have larger/deeper urine collecting basins vs the NH and airhead, did you maybe forget to insert that separate piece between the liquid and solid section, it creates a ridge between the two? Also, I know with the first batch/release of Separett toilets the paddles weren't functioning correctly, so there was a modification created to improve the functioning of the paddles/flaps based on feedback from customers.
Anyway, we are still on the fence re which one we will go with, but for all the ladies out there, here's a pretty good review of the Separett Villa from a female user.