I have a Dx6i and a Dx8. I haved helped a couple of people with the Dx7s. (Do not confuse the Dx7s with the older Dx7. The Dx7 I understand is a programming nightmare, the Dx7s is a dream!) I find the 7s and the Dx8 are extremely similar, you just get the extra channel and I think telemetry on the Dx8. (Don't quote me, just going on memory about the telemetry. Maybe the 7s has it.)
There is a big jump from the 6i to the 7s/8. The 6i is good for starters, but I found after about 3-4 years, it couldn't keep up with what I wanted to do with my models. I could do the basic 4 channels, flaps, and retracts. Or if you split the ailerons for differential throws, now you used 5 channels for the basic 4 functions, and you're left with retracts and no flaps, or I'm pretty sure flaps only on the gear channel, the way the 6i works.
You also don't have as many mixes available. With the Dx8, I was able to do this to my F-35 EDF: The stabilizers were setup as elevons. The ailerons work as spoilerons, and their aileron function can be turned off with a mix switch. Sounds basic,(Well, actually a bit overboard for a model like this...) but you'll never come close with a 6i. I had to use a lot of mixes to do this.
There's also the benefit of 3 position flap and rate switches, as well as speed control for the flap movement.
If you really know you're staying with it, I'd go with at least the 7s. That will take much longer to outgrow.