I think it depends very much on the length you'll have to wait at immigration, if the plane is early or late, the weekday you're there (traffic) and the timetable of the Maglev.

Also, keep in mind that Shanghai is under massive reconstruction in the Bund area (Nanjing Lu) at the moment. So, it's not as nice as it's usual at the moment.

IF (big if) you're lucky with the plane and take only minutes at the immigration and then hop on the Maglev realising it will just leave, you can pretty much make a VERY quick trip to the bund on Saturday/Sunday. I won't risk it on a MO-FR, but there are other nice things to see less in the centre of the city, you might pick instead.

If, on the other hand, the flight is a bit late and/or immigration takes long time (I've had anything from 5min to 50min in the past) and you find out the Maglev will not leave soon (They aren't running it that often) than it might be smarter to stay at the airport (or only do the Maglev trip, which is nice especially on a sunny day)

In the end, according to my experience it will be smartest for you to just decide what you'll be doing once you've cleared immigration and checked train timetable (or do this in advance)

Shanghai isn't the worst airport, so staying there a few hours won't be that bad compared to other airports where you just want to leave them for whatever short time you get.