The S90 is the most comfortable cruising car Volvo has ever made, and possibly ever will make. It's a quick decision on which would be a better highway vehicle. From experience, it also gets better mileage than the early S70 w/IL5 engine and an auto trans. My 960 gets 27 mpg doing 85 all day long. The S70 is hard pressed to get 22 mpg and rev's higher at highway speeds. The S70 has FWD and would be better in the snow, although the S90 w/the winter/wet mode is almost as good as the S70.
The S70 of that year (pre-2000) had a lower reliability rating that the later ones and would not be much different in maintenance costs than the S90.
All that said, go with the lower mileage one, the S90 and you can get a pretty good deal on an extended warranty with the low miles to cover any hidden repairs that pop-up. This is not usually a big consideration of most Volvo buyers since they last a long time, but if security is what you want, you should seriously consider the ext warranty. I got my 960 with 17k miles and a 6 yr/100k warranty that is still valid until Jan '06 or 117k miles (now w/77k mi). This has saved my financial butt many times.
If the S70 is a n/a car you'll be impressed at the improved performance of the S90. It is a much better car all around.
--
'89 245 Sportwagon, '96 960 sedan