There is very little I can add to the advice above about the best second airplane. What your "ultimate" plane is going to be, will have a big impact on which one you should get for a second plane. If you are looking for wild 3D go for a straight wing (Something extra, stik). If you are looking for a scale, or semi-scale warbird, then you can go with one that has some dihedral (fourstar and the like)--unless by scale you look at the Extra 300, Edge 540, etc, then get a straight wing. Unless you are a VERY disciplined pilot, that takes things one step at a time, and never goes beyond your "envolope" take the advice of getting an intermediate airplane before "stepping up" to something else. If you are NOT a disciplined flyer, choose something you think looks really good and that you really like so you will be more likely to repair it. :-).

The advice about keeping that trainer is also good. I had three different planes but always flew my trainer for the first--and many times, the last--flight of the day. When I crashed my other planes, I still had my good ole trainer, with the contols maxed out. The trainer can also be quite a challenge on those windy days, but don't forget to put on extra rubber bands (disregard that if you have already converted it to a bolt on wing). I never did get around to converting it to a tail dragger though. I did increase the size of the elevator, rudder, and ailerons. It just seemed like the thing to do during some of the later repair sessions. I hardly ever get the shakes flying it because I know that even if I "lawn dart" it (which I have done) I can fix it and be flying again next weekend. I am not at all afraid to fly it in strong winds, while my others sit in the car. If you learn to do everything (and I mean everything) with a trainer, then there is little need to have an intermediate airplane. The reason they are recommended is because so many people want to do it all now, that they don't expire the possibilities and capabilities of one plane, before moving onto another, and that put the people ( and their planes) they fly with in danger.

As many others have said, the most important thing is landing! Practice that a LOT, get it down pat! If you can land well, stay two mistakes high, and don't continue shaking after a big mistake, you can fly most anything. And, if it is all balsa and ply, it can be fixed after even the worst crash--just stay away from the pits and inside the alotted flying area, so you don't hurt anyone. Don't give up on your trainer too soon, they may be capable of a LOT more than you might think. Some of the moves (i.e., knife edge) are more difficult, but they are possible. If you can do it with a trainer, the others will be a piece of cake. Get the plane that you will be the most happy with. If you put the safety of yourself and others first, the people you fly with will like you even better. :-)