I've read the flex is better for right after the return pump because it makes maintenance a lot easier. Otherwise I think I'd do rigid for the overflow and for connecting to the bulkheads just because I would trust it more, and I think it would look cleaner.
PVC. UV and salt water tend to make vinyl brittle. no longer flexible at the very least. I personally like hard pipe PVC because it is dark in there and won't grow algae and i like the clean look of hardpipe. but for flex after the return pump for vibration absorption i prefer silicon. It stays flexible and doesn't stink like harbor freight.