I'm running a circular 100mm vent from above a shower to an external outlet. Total distance is about 5m. Most of the run can be done in 100mm rigid plastic, however there are two corners that aren't standard angles and I need to bridge across them using flexible ducting. What are the relative merits of using plastic versus aluminium flexible ducting? The amount is so small that price isn't an issue - I want something that will give least flow resistance and last longest (it's buried inside a ceiling, so a leak would pump vapour onto the joists).