Well, flex pvc is said to dampen vibrations thus a lower noise decibel. If you use flex make sure you use a good amount of glue and push the tubing all the way in, holding it there for at least 30 seconds to a minute. Because of the somewhat malleable nature, the flex pvc tends to want to pull back out otherwise. It does allow for eliminating varying turns in plumbing so I will usually use it for direct connections to a pump or from overflows to the sump if applicable leaving the rest to be hard plumbed. Make sure you have 2 union-valves in conjunction with that pump, 1 between sump and intake and 1 for output.