Sims are great for beginners and advanced aerobatics, there is a lot of stuff in between that you need to learn on an actual plane. I have seen guys got right to big aerobats and never understand that you have to use you left hand to do little things like throttle management so you don't blow the ailerons off which the sim doesn't teach you.

I have also seen guys go from trainers to overpowered warbirds cause that's what they wanted to fly in the first place and 3 or 4 planes later start to figure out about tip stalls and elevator induced snaps.

When people ask "what's a good second plane?", they usually want someone to tell them it's OK to get something that might be over their ability so they can justify getting it. If you don't agree with them they go find someone else to ask.

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. After that get the warbird or funfly or whatever floats your boat and you'll have a better foundation to build on.