I agree that maintenance history is key. I would say that of the '95-'98s, the '95 would likely be the most problematic as I believe some have engine wiring harness issues (updated in '96). An S90 will have different seats, a different center console, and the '98s have an outside temp sensor for the climate control that the earlier ones don't. If the S90 is an "SE" it will have a suade and leather interior, unique wheels, painted lower trim, tweeters in the b pillars, and a wood/leather steering wheel rim. IMO if you find an SE in excelent condition that would be the car to have. Apparently the '98s have a softer ride, and my V90 does ride softly, but I haven't noticed any suspension part number differences yet. I like my '94 960 too, but it's quite different then the '95+ version. It drives quite different and looks different inside and out. I like to call the '94 my "old school" Volvo :) Both are great cars, but watch for A/C problems. My '94 needs a new A/C compressor and the evaporator core in my '98 V90 is leaking badly :( Make sure the timing belt pullies are replaced along with the timing belt unless the car has very low mileage. Do your reasearch before buying and be willing to pay for a nice example.
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Chris. Halifax N.S. '98 V90 > 200K km, '94 964 > 91K mi. SOLD: '91 745T and '91 745 NA (I miss both of them!).