the corsair or t-28 would both be good second planes. i second using faster models in the sim, it will help build reaction time and instincts. the super cub is also a good choice as it allows you to fly in a little more wind, and get more wind compensation flying in in trainer form.
all that said, i HIGHLY suggest you invest in a better radio than the champ, preferably something with model memories so you dont have to have so many transmitters around. i suggest getting either the DX6i or the DX7 (10 and 20 model memories respectively) futaba is also good, but since you already have a champ which can bind to the dx6i or the dx7, thats why i suggested those two. that and the fact that hobbyking.com now offers EXTREMELY cheap spektrum compatible recievers (if you buy these from hobby king, put a dab of hot glue where the antenna(s) meet the PCB circuit board - they have a tendency to break there)
if you go with a super cub LP, get the bind and fly version because you can use it with your champ transmitter AND a DX6i or DX7.
another plane to consider, if you think you are up to building with foam, is the "Big Easy". Link
you could probably build that plane for less than 50 bucks, and then with electronics (including a spektrum compatible reciever from hobbyking) from hobbyking, you might be up to around 100 bucks. that big easy plane seems nearly indestructable from the videos on youtube. he cartwheeled it on purpose with almost NO damage, he lands in tall grass on purpose, zero damage there, etc. watch the videos on the link i provided above to see him crash test the plane.