ORIGINAL: BasinBum
Trainers fly like crap for the most part so after you use them for basic flight (which includes rudder manuvers) it's good to move on to a low wing semi-semetrical airfoil to learn inverted, basic aerobatics and the such.
I agree BB, but let's not leave out the semi-symmetrical high or shoulder wing.
While I am a big fan of Great Planes, I think their claim that the US 40+ is a good second airplane is nothing more than a blatant marketing pitch aimed right between the eyes of gullible beginners.
Here's how I see the progression in a nutshell:
Trainer: Learn basic flight controls, so you can take off, fly around, and land without incident.
Length of time: AS LONG AS IT TAKES. (I have had 1 student solo his first day out, I have seen others that have been flying for 20 years and still need someone standing next to them.
Intermediate Trainer: Start learning to enjoy flying without the restrictions imposed by a trainer. Learn Basic aerobatics, Crosswind landings, and some of the other more advanced techniques.
Length of time: AS LONG AS IT TAKES.
And finally, a word about your "Second Plane". I have said this before, and I'll say it again: You are more likely to crash your second plane that your first. Airplanes that are built extra light, (like the UCD) break extra easily.