I remember when I was in the market for my radio-  The guy at the hobby shop asked me (same as people here have responded), "What do you WANT to fly?"  I thought about it for a few seconds and realized that I really want to fly warbirds.  He then asked, "What kind of control surfaces to you want in your planes?"  With all the cool things you can do with an RC model that was a little tougher to answer but I finally concluded that I would only need 6 channels- throttle, elevator, ailerons, rudder, flaps, and retracts.  I was looking at Spektrum radios so the way I answered those questions made my choice easy.  I bought the DX7.

I figured that since I really only wanted 6 channels I'd get 7 so that I could have the flexibility of that extra channel for lights, or maybe an opening canopy, bomb drop, etc. 

I guess the point I'm getting to is what most people have said already-  Ask youself what you want to fly, NOT what you're flying now.  Do you want to do scale flying, or 3D?  Do you see yourself sticking to park flyers, or going up to 40 maybe 60 size planes?  Or are you wanting to get into the BIG model?  All things you should ask youself.  I've gotten endless hours of fun and have had an awesome time with the 7 channels I have yet to need them all.  I do have a DX5 that I picked up for a killer price as a "backup" but it's just that.  If I ever decide to go deeper into the hobby by going to large sacle, or maybe jet, then I'll pick up a "bigger" radio.  Honestly though, that part of the hobby doesn't interest me as much as just throwing a 40 size sport plane or warbird in the air and flying approaches, which I think is the most exciting part of this hobby....landing.

My advice, pick up a DX7.  It'll be more than enough radio to grow into, and if you outgrow it then spending the $$$ on a bigger radio won't be an issue- cuz you'll probably have spent alot more $$$ on the planes that will need that bigger radio.