I don't know about the demo you're talking about, but I do have a 688.
I've used a multitude of the portas, from 414s to 388s (the 1/4" 8-track with mixer built into a console).
Most extensively I've used the 488MkII (to record/mix 3 demos) and the 688 (so far just one indy release on it).
My own experience is that the 488MkII might have some minorly improved electronics inside, but its lack of other niceties that the 688 provides doesn't really make it a comparable unit.
The main differences (to the average user) are:
488 - 2 mic. inputs (with phantom)
688 - 10 mic inputs (no phantom)
488 - 4-bus router/mixer (4 tracks recordable at once)
688 - 8-bus router/mixer (8 at once)
488 - pushbutton bussing
688 - electronic bussing
488 - No track outputs; stereo out ONLY
688 - individual track outs (so you can copy up to ADAT or similar)
The learning curve for the 688 is definitely steeper (takes 30 minutes to learn rather than 10).
The 688 is slightly larger, slightly heavier, and a hell of a lot more robust/tough than the 488.
The tape transport is essentially the same. The specs on it are identical (as far as I've been able to find). You can take tapes from one to the other without hassle.
Now, considering these main differences, and assuming you can find one in good shape (shouldn't be hard), I'd choose the 688 over the 488. Of course, you can get a digital unit for not much more, but then you're (usually) stuck with a mixer where you can only play with a couple of tracks at once and such.
I've never had any serious complaints about the sound quality of any of these units. They may not have ruler-flat response from 20Hz-20kHz, but they still sound damned good. In fact, when I saw the 488MkII for the first time, I groaned - then I used it. It blew my socks off. The 688 is no different in that regard. You'll find yourself wondering, "How do they do that with CASSETTE?"
Anyway. Hope that gives you some idea. If you have any other questions, please ask.
D.
[This message has been edited by Dex (edited 05-21-2000).]