S90 is supposed to be more reliable than S70; yet at being 13-14 year old cars, previous upkeep is more meaningful to consider.

S90 is a very different car from S70. They drive very differently. S90 is a very nice highway crusier, S70 is better suited for drive-around and fuel economy (except Turbo or R).

S90 was the end of an era (RWD, reliable boxy-looking, grandpa series), the other the beginning of another one (sportier FWD cars). Although S90 has the luxuries of the newer cars and the RWD and reliability of the old Volvos.

They have very different technologies. S90's rear for example rests on a single leaf-spring, fiber-glass monoleaf mounted transversally, wheel to wheel. Same monoleaf design as Corvettes. Since S90 is IRS (independent Rear Suspension) Volvo used the monoleaf to lighten the load for better handling.

S70 has regular coil springs.

I-6 is a lot more stable inherently than I-5, so you might not have the problems with engine mounts. Transm is basically the same, yet due to engine refinement, S90 transm should outlast S70's transm. Check to see if S90 has the locking Diff. You can tell if S90 has the cold-weather option, by looking for the snowflake symbol, next to the outside temperature display on dash.

You won't have the axle problems in the S90. PCV is similar. S90 is less forgiving with delaying the Timing Belt interval. Belt just drives a longer drivetrain and it's more crucial it's changed on-time. S70 is more forgiving on the Timing Belt interval.

S90 will not have the ABS Module problem. Both have mechanical Throttle control, so if you go 1999-02, you know what you are faced with.

If you drive in snow, I prefer my RWD Volvo on snow-tires than my FWD on snow-tires.

Having owned both RWD and FWD Volves, my V90 feels like the traditional Volvo, while my 850 felt like a ricer. Even doors thump differently.

Besides wheels and a few other parts, most parts are not interchangeable.
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1998 V90, 151k miles

I would expect the two models to be similar in durability and maintenance...maybe a slight edge to the S90 due to beefier transmission, lower-stressed engine, no CV (FWD) joints or complex front hubs. Make sure the engine idles silently, no tapping noises. My son has an XC70 with the 5-cyl and my S90 feels smoother and quieter under acceleration.

Both cars are somewhat sophisticated electrically - lots of power equipment to go wrong. But my S90 has behaved well.

The engines are very similar so timing belt maintenance is a must. It looks to me like the S/V70 engine is basically just the S90's 6 with one cylinder cut out in the middle of the block.

The 1998 S90 has 181BHP compared to the S70's 165(?) or the LP Turbo's 195(?) so there won't be a big performance difference. Fuel economy should be close, I get ~27MPUSG at Interstate speeds.

Best thing is to test drive it to see if the car "grabs" you.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.