Can't go wrong with either but I sold QS and kept SW. When I did a stack ranking of all games vs. all other games I have Seawitch was the top classic Stern behind Meteor (my all time fav).
I really like shooting drop targets though, and I like the rule where you shoot all but 1 down on SW to get it blinking for the collect bonus shot. That's the key to high scores on that game IMO. Also it's just fun to backhand the top bank from the lower right flipper. I never try for the spinner or the riptide looping shot, it's not worth enough.
QS is a heavy left spinner game, and most people I've seen play it play it wrong - the strat I'd use is get the center 4 bank to 20k per target and go to town there (hit the eject hole to advance it - not an easy shot most bounce out)
There should be a sweeping bonus added to QS that I would probably do if I every got around to it.