Hi David,
My impression (from what you told) is, S670 could be more than enough for you. I had it and can honestly say, it's a darn good keyboard. Actually, it can do "99%" of whatever SX700 can do (that much about keyboards progress) and you can practically deliver almost the same result. That is, S670 is rational "common sense" decision. The question is, how important is "1%" difference for you?
In my eyes, main difference (in resulting performance) is:
1. S670 doesn't know "S.Art" voices. In my opinion, 90% of S.Art voices don't really sound that good in real use and I usually decide for some other voice type instead. But!.. the rest of 10% is quite amazing and can make difference in end result for sure.
2. S670 can "only" layer 2 voices for right hand (that is, by pressing on key, two instruments play), while SX700 can layer 3 voices. I mostly use one or max two voices at the same time (to avoid music being too "busy"). But for certain kind of music (i.e. heavy Classic orchestra, complex synth genre music, etc.) it could be a welcome feature.
As always, it's not only what you can do, but also "how" you can do it. And here S670 really can't compare to SX700. For an arranger, S670 just has too tiny screen to do "serious" work. Yes, everything can be done on it.. but is so uncomfortable. I see S670 ideal for beginners/students where they can learn and get the "taste" of arrangers.
S700/S775 vs SX700... Main advantage of SX700 is (as I see it): 3 layer voices, better panel buttons, bigger user memory and touch screen. And only because of touch screen, I wouldn't even consider S770/S775 (in case you wish to "dig" into arranging seriously). Yes, being the latest, SX700 is a long term decision.
Hope that is of some help.. think twice, buy once