Having driven quite a few 928’s and 8 series I can offer you this:
In terms of pure grunt you would need an 850 csi to stay anywhere near a 928 gts in a straight-line sprint.
On a motorway the 8 series is much quieter and smoother, but should you stray off it on to some A and B roads the 8 feels big and cumbersome whereas the 928 is for want of a better word, spectacular.
If you opt for one of the V8 8 series the running cost will be significantly lower than a 928.
If you simply want to cruise along in peace and comfort go for the 8, if you want excitement go for the 928 preferably with a manual gearbox.
Finally you might consider a jag xk8/xkr. Their cabin is much nicer than either of the others, they look nicer than the 8 series and they have less of the desperate mid-life crisis image that comes with a porsche.