1) build quality
2) ride quality
3) 2nd hand price
4) possibility of modifying
1. Saab is on-par with German vehicles in terms of build quality and reliability, perhaps better than most late-model Mecedes and Volkswagens. The 9-5 is considered to be one of the most reliable European cars out there. Volvo is sliding in the quality department, and the S80 is a POS.
2. I haven't driven many Volvos, but the last one I drove was the old RWD 960. Comfortable cruiser, decent power, but very boring driving experience. Saab on the other hand, it's all about the four-cylinder high output turbos.
3. Saab has pathetic resell value compared to Volvos. I think with the massive rebates to push these cars off the lot new has taken a big hit in depreciation. Otherwise, Saabs are an excellent used car buy.
4. Aftermaket parts for both makes aren't as huge as the German makes, but they are out there. Saab has Abbott Racing, Hirsch Performance, Nordic Tuning, just to name a few. Volvo has IPD, Evolve, etc.
I'd pick Saab > Volvo, because as much as I like the look of recent Volvos, there are way too many of them out here around these parts and they seem to be all driven by the granola bar-eating, Birkenstock sandal-wearing, NPR-listening types. Not that I have anything against that, but I think Birkenstock sandals are ugly. And Saabs are more fun to drive.