I know you got a lot of responses already, but as was said, expo is a way you can dial up or down the sensitivity of the sticks around center position. Comes in handy with twitchy planes like 3D models.

3D is a type of flying post stall. You are really flying off the engine power a lot of the time. Aerobatics is a type of sport flying that usually consists of the general loops, rolls, split s, etc.

It all boils down to your budget. The DX7 you chose is a fine system.

For me, a 6 channel is all I need in terms of channels for the type of models I fly, which is 3D and general sport. A 7 channel or 8 channel would be nice for the added mixing and triple rate switches and I might upgrade some day, but it's not a requirement for me. Right now, if I'm going to spend $600 on a radio, I'd rather spend that money on another model.

Feel free to PM me if you need any help getting started.