I'm British and I actually pronounce it somewhere in between those two - as /ˈres.trɒ̃/
I've certainly heard both of those very often, and everything in between. I think it's one of those borrowed words that we still haven't quite got our tongues around! Perhaps it's an indicator that people seem to try to avoid saying it, often just by missing it out altogether ("shall we go down to that Italian on the high street?")