Weight difference between S970/S975 and S770/775 is not worth to mention: it's only 0,5kg -its' a difference you won't notice at all. So, buying S775 only because of weight difference really makes no sense.
However, S775 is quite cheaper compared to S975.. even more if you get some S770 second hand. That way, you can maybe afford having a "spare" keyboard. Question is how much will that spare keyboard be used for rehearsal... if it's worth the investment for mostly standing there?
There's another option you've mentioned, but only you know the answer: how much of S970 advantages are you using at rehearsal scene. Is VH a must have? You need to have hundreds additional voices and styles installed all the time? And finally, you think you can do rehearsal without S.Art! voices (using Regular voices instead)? Well, S670 is dirty cheap nowaday and is MUCH lighter in case you decide to carry it.
Take time and look at this video to get an impression:
https://www.youtube.com/watch?v=EGgjhVXw5YAAnd no, sound of S670 is not of lower quality compared to S775 (I have both). It's just the more you pay, more options/features you get. Some might be essential to you, and some you might never use.
I hope this writing is of some help to you.