I always recomend heavy wall flex liner but i honestly feel that any liner on the market is more durable than the lightwall 2ply smooth wall stuff.
Is this the flex King Heavy Duty that you are talking about? Would I be nuts to look into rigid pipe? I have a challenging install once it gets close to the stove and not sure I could make it work without the bendy flexible stuff. That said I'm in the camp of spending money once even if it's expensive.