The Wirbelwind seems a little deadlier on targets actually trying to strafe you, they can sometimes weave between the rocks the Osti belches out, but they can't dodge the 20mm firestorm.
On crossing deflection shots, it seems to get pings where the Osti might get a kill. Often enough of course, the Osti would miss entirely. One thing about an Osti, one ping, one kill.
I suspect the Osti will remain more effective on buffs at 1.5+ distance.
But the Wirbelwind should be an easier flak to use for the average player who seldom practices in guns.
Still haven't decided which I like better.