Generally speaking, the 70 series is a less jittery ride than the 850, but they are for all intents and purposes the same car, as you probably know. As noted by spratt1, the 98's were a more trouble prone year. If the car has enough miles on it, however, most of the 98-specific nuisance items will have already been fixed, specifically:
  headlight connectors (recall)
  front turn indicator connectors (recall)
  ABS unit (usually dies around 70K miles, part $416 plus labor)

The 99-and-later S/V/C70's equipped with TRACS have a system that doesn't cut out at 25mph, if traction control is useful to you.

Me? I like my 1998. A lot.


--
David
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98 S70 T5SE // Black, misc mods (mostly lighting), red calipers
92 940GLE // Hella MicroDE foglights