go with stainless!
If it was a direct-fired burner exhaust flue, I'd say galvanized is fine. But not with a kettle steam exhaust. Go stainless. I know you have a drip ring, but that won't keep your galvanized from rusting soon. And if you have a long vertical run, you will need some kind of expansion joint to keep your steam pipe from heaving your flashing at the roof. Do it right the first time, and you won't have to do it again. And you'll sleep better. Cheers & good luck!
Palau Brewing Company