This is a tremendously hard one for me, because I personally LOVE Qlab. I'm close to buying a (used) Mac for my theatre out of my own money just so I can use it (we're an all-HP company due to Greek Mafia-related reasons...)
But which of the two is more important to me as a Stage Manager? In theory, a stage manager wouldn't necessarily be running cues themselves, so is Qlab really a contender for a Stage Manager's tool? Tempest is absolutely right, there are times when Qlab enables us to do a production so much more easily than the alternative, but like BayAreaSM I think that Excel is always useful and more directly applicable to stage managers.
It kills me to do it, but I'm voting Excel. To me, Qlab is that bad boy with the awesome car that all the girls want to date, but Excel is the chunky accountant who may be boring but at the end of the day he's what you really need.