Does it matter whether the mag breaks, or if the gun malfunctions, or if you expend all your ammo and you need to keep shooting? Either you have the alternate capability on your hip or you do not. It is cheap, simple insurance in the case of a spare magazine. It is expensive, more complicated insurance in the event of a BUG. However the BUG allows you to fix the gun grab problem, where a spare mag does not.
As far as probability? Well, they're both (fixing a gun and making a 25 yard shot on a dude) statistical outliers, but they're still on the bell curve. I say work at both but don't spend an inordinate amout of time on both.