There's a whole lot more information I would need to make a recommendation. I fly with a DX8 and I love it. I wouldn't be happy with anything less because I am using the features it provides as well as all 8 channels. I have friends with the DX6i so I'm very familiar with it and the old DX7 but not the DX7s. I can tell you a few things about the DX7s based on my DX8 though. Here's a few things I need more info on.
What type of flying do you do? I know you said some 3D but are you doing indoor profile 3D or big gasser 3D?
How many channels do you use on your current models?
What types of models are you looking at for the future?
How many models do you fly with?
Do you scratch build models or stay more with ARF's and RTF's?
Here's a few quick things about the DX's that I've found.
The DX6i is a very very capable radio that my good friends fly with most of the same planes that I fly. My DX8 does everything the 6 and 7 do plus more. I spent the money to have the extra channels so I had a radio I could grow with. I'm not disappointed. There are lots of little things that the DX8 does like the DX6i and DX7s only have two position switches for rates, gyro, governor etc where the DX8 has three position switches. This is huge for me as it allows me to have three settings at my fingertips so It makes tuning a whole lot easier. For my super fragile 3D indoor planes I have everything set on my flight mode switch. I have a very tame setup for taking off and landing, my normal rates for most all flying I do and then I have it cranked up to max rates when I feel like pushing myself, all by flipping one switch. Now, the other two also have a flight mode I believe, but you only have two settings. This is not really a bad thing not to have, but a great thing if you have it. I wouldn't buy a radio based on this alone though.
Another thing that we found the other day when setting up throws on 3D planes is that I have the ability to adjust my max throws on each servo in each direction independently of each other. So, I can get into my radio and set the rudder throw to 130% to the right and 105% to the left to get the maximum throws. My friends DX6i can also adjust the travel, but each direction is linked so he has to set it up both at the lesser amount. I hope that makes sense the way I'm saying it. Again, it's not a huge deal if you don't have it but it's really nice to have. I can compensate electrically for things he has to adjust mechanically with no disavantages.
A huge benefit is the screen. The backlit screen is nice and all but that's not what I'm talking about. The bar graphs showing the position of all the servos is awesome. I'm not sure if the DX7s has it but the DX6i does not. It has the roller interface and basic screen but you have to do all your throttle and pitch curves, direction, travel, expo etc all by the numbers like a Futaba. The DX8 has graphs that show you pretty much everything. I was doing some complex (for me) mixing of 4 servos in a combined 4 aileron with flaps setup and i was able to do all of the programming right off the screen and get all the servos moving the way I want, adjust for direction, travel, and make sure I wasn't over driving any servos without ever having any of the pieces in hand. It also has graphs for pitch, throttle and expo that make it waaaaaay easier to understand. I use my radio to help explain the settings that my friend uses in his DX6i.
Does any of that make it worth it? It really depends. I'm sure there's a whole lot more that the more expensive radios can do, but my friend with the DX6i flies just as well as I do.....lthough we all agree that my radio is a lot more comfortable and the adjustable length sticks are great. My vote is that if you can afford it and you think there's a chance you are going to need more channels later on, I'd get the DX8. I use the extra channels on my complex foam models and my bigger nitros and helis. I use all the channels but if you don't, the features may not be worth the money.
One big feature is the model memory. The DX6i can only hold ten models and you don't have the option of using an SD card to save more. The DX7s has an SD card and a 20 model internal memory, the DX8 has an SD card and a 30 model internal memory. I already have fifteen models on mine, so there's no way a DX6i would work for me, and chances are I would use up the 20 on a 7s by next year.
The SD card options also give you the ability to download setting from the Spektrum online community or share them with local pilots. I used the programming from an expert at the hobby shop to help understand how I needed to program my heli. I saved it in a slot next to mine and would use it as a reference point while learning to setup my gyro.
Looks like the DX7s also has the telemetry options which I think are cool. I'm going to put some sensors on my nitro heli for rpm and temp. The 8 is quite a bit more, the 7s is only a hundred more than the 6i. My vote is forget the 6i and go for at least the DX7s. I think the easier programming and extra model memory are worth the extra money, plus the comfort items. The 6i is a good radio, but for the extra 99 bucks you get a whole lot more radio.
There's my 200 cents.