The answer is both
From my experience,you'll have more fun playing scenarios than QBs.The scenario maps present more challanges and are more detailed. I guess it's obvious with the naked eye that more work has been done by the developers to create the scenario maps than the QB ones. If i played a scenario once and i'm about to play it for a second time,i prefer to let my opponent know i'm not playing it blind,so he can have a look at the map/OOB. I guess this practice pretty much nulifies any (big) advantages i might have from playing it previously.
On the other hand,the QBs tend to be more balanced. There are few stock scenario designed for H2H play,so usually,one side will have the upper hand right from the start.Which doesn't happen if it's a QB.
All in one,i like to have half my games as Qbs,half as scenarios. For my,it's the perfect combination of enjoyment and balance.