That's a tough order, Danny! They are both wonderful instruments. Both have 50w amps internal (25/channel). I Haven't had the 7x for awhile now, but if I recall, the 7x had something like 50 orch voices or so, the 8x has 180, I think. Both are expandable with expansions like Dallape, Alpine, AccOrch. The 7x will take up to 2 expansions at a time, the 8x can take 4. Both have great sound, but I think (and it's my opinion, not fact!) that the 8x voices are cleaner, maybe a bit clearer. Both will allow up to 100 sets, and both come with 37 or so of them already set for different types of accordions. Both come with a version of Roland's tone wheel organ voices and on both the organ voices can be customized with virtual drawbars. Both have a port for a USB memory stick, both allow recording true stereo right from the instrument itself ( records in wav format) and both will read wav and mp3 audio files.
Both being really good instruments, why did I decide to go with the 8x. Well, it wasn't weight, because the 8x is a bit heavier than the 7x. Since I don't stroll, for me it wasn't a factor. But the 8x had some differences which made me switch:
While both can use sets, with the 8x I can create specific user programs, and then make a "User Program List" which is a list of programmed sets I have created for specific purposes. For example, I regularly gig with a jazz/blues harmonica player and I have made up a user program list specifically for using with him, have all the voices and effects pre-set so I don't have to remember what voices and effects I was going to use with what kind of song, which of the 100 sets they reside in, etc. I have an anniversary gig coming up and will make up a User Program List of sets I create for specific songs that they have requested. So basically, you can have a Wedding List, Lounge list, a gig-specific list, etc. I find it useful and very quick.
The bellows resistance can be changed on both the 7x and 8x. The 7x controls it by a wheel on the bass panel that changes the size of an opening for air to escape. With the 8x, they have tried something called Dynamic Bellows Control (which you can switch on and off). It tries to more closely mimic the bellows feel of an acoustic accordion. On an acoustic, the bellows resistance changes with the number of keys pressed, for example, since more valves are open. On the 8x, the Dynamic Bellow Control, which is pretty adjustable, attempts to re-create that. It seems to work pretty well, but if it were the only difference between the models, I wouldn't have switched. Many 8x users are so used to the 7x bellows feel that they turn Dynamic Bellows Control off.
The 8x USB port for the memory stick in on the front grill, on the 7x it's in the back requiring you to pull back the bellows pad and open a little door.
The 8x has a built in port for connection to a computer, which I am now experimenting with. Using a version of PrintMusic from Finale, I am able to record and render a simple piece of sheet music in real time. My computer recognized the 8x's computer port immediately which was nice. Haven't done a lot with it yet.
The charging circuitry is built in on the 8x, so I don't really need the large pedal board to charge it. There are for the 7x, by the way, chargers people have designed so you don't have to use the pedal board to charge it. I think Jim D has some of those.
The 7x midi ports are on the pedal board, although you can use a special cable to allow midi in and out without the board, the 8x has the midi ports built in.
The 8x has 3 chin switches at the top of the treble keyboard. We think this is why the 8x keyboard is thicker than the 7x. I have found the chin switches useful (never had them before!) for doing things like modulation of guitar sounds and sustain, as well as many other functions.
The number of orchestral voices (treble) available at any one time in any set on the 7x is 28, on the 8x is 56 (8x has orch1 and orch2 switches), and 28 organ settings at any one time. Most people wouldn't use nearly that many, but it is neat to have the option of so many to pick from on a single set. On both, you are limited to 7 orch bass types and 7 orch chords types at any one time in a set.
On the 7x, two orchestral voices can be layered, and layering is available on the treble side only. There are more layering options on the 8x. For example, you could layer orch1, orch2, and accordion on the treble; Or you could layer Orch2, Organ, and Accordion on the treble (Orch 1 and Organ share the same setting circuitry so you cannot layer Orch1 with Organ). The 8x will also allow you to layer sounds on the bass side while the 7x doesn't, I believe. so you could have different combinations like orch bass and accordion chords, or accordion bass and orchestral chords.
The 7x allows you to have a split treble keyboard. One of my programs has accordion in the lower half and any other sound I choose in the upper half. I don't believe you can split the keyboard on the 7x.
Both 7x and 8x have drums sounds available. The 7x comes with 1 drum kit for the left side only, the 8x comes with 15 drum kits which can be used on either side. I do a nice drum solo on It Don't Mean a Thing using the treble keyboard.
Finally, there are two things only the 8x has. The first is a looper which allows you to record up to 1 minute and play it back continuously. That's good for songs that have a repeating bass line, for example. And it allows overdubbing so you can add more to the loop. Richard Noel has an interesting looper demonstration using his 8x on YouTube. The second is the 8x has an FX processor which allow for things like tap delays, echos, flanger, phaser, etc. This can make a lot of music sound pretty cool.
The 7x is a great instrument; If the 8x hadn't come out, I'd still be playing it and would be very satisfied with it. It served me well and was extremely dependable. So, if you are kind of deciding on which you might get, it all depends on what you do and what you want available option wise. If you stroll the 7x is lighter. If you don't want or need the additional bells and whistles of the 8x, the 7x will do just fine. If budget is a problem, you can find some used 7x's at a pretty good price.
It all comes down to how you use your instrument, what you want to do with it. I've been typing so long I forgot what the question was