I'll second the Mid-Star suggestion. I did it as a taildragger and it works great.

One thing no one mentions but is VERY important in a second airplane is for it to be tough. It needs to be able to take quite a few hard landing, dives at full throttle, etc... You know how to take off, land, etc.. But the second plane is where you really learn to FLY. I would look for one without "wire" gear, easy to access engine, etc..

I think both the 4*40 and Mid-Star match this and both are easily modifiable to do whatever kind of flying to want to do. I just lean towards the Mid-Star because I don't like the looks of the 4*40 (bubble canopy yuck).

Cut tips and 2 bays off of eash wing side, lengthen rudder, ailerons, elevator, put two servos in the wings and I'm told the Mid-Star is one of the best acrobatic plane trainers btw.