RE: Ultra Sport decisions...
With the 70 four stroke engine you plan on using I would go with the US40+ I have one and it would be great with that engine. The other US you mention are more suited for two stroke engine and faster flying. I have a US 40+ that I lengthened about 3" in the rear and mounted a Super Tiger 90 on the front. It flies great at high speeds or slow. I built one stock about 5 years ago and it flew well also with 46 engine.