i had a magnaflow in my old 93 Z28. Its about best sound you can get for less then 500bucks imo. It had ZERO drone, and fits great, and with stainless construction, it look nice too.
Flowmaster i think would rust, not because of rain, but from constant heating and cooling. And moisture in the air. My Rustmaster in T/A is showing rust around welding already, and it have about 250miles since i install it(and that include no driving in rain)
I know that Magnaflow is almost twice as much. But its worth it. Heck if T/A was trouble free, I would have gotten one for it. But with all the issue its having, i didn't feel enough love at the time to spend that much for it at the time