Which MB "320"?

There was C320, E320, CLK320, SLK320... I presume you meant E320, which has been replaced by E350 now...

So why does MB sell whatever 320 higher than S80? The short answer is because they can. They just have a different demand curve than Volvo.

In terms of differences, the biggest one is that MB mounts the engine front/back driving the rear wheels first, while Volvo mounts the engine left/right driving the front wheels first. In general, front/back/rear set-ups are thought to lead to better handling, but you may or may not care about that... Only a test drive will tell.