Oh yeah, since you're on a bus you've got civvies in danger.
I'd still go for 'drop & pop'.
Your shoes do the best in defending against a razor attack, keeps the distance and gives you two additional weapons (just don't shoot yourself). Also, when you shoot from your back, your shot is angled upwards so stray shots & penetrations go up & out of the bus' roof, not into a packed bus.
Opening an umbrella in the aisle is gonna get you tangled up. and since you can't see him, how will you know he isn't reaching for his Tec-9? He may even decide to slash up a few grandma's nearest him since there's an open umbrella between you two.