Cuff dumps are simple to use, arm up air out- easy. The problem is when you start doing other stuff underwater, photography, wreck penetration etc. Its not always convenient to have air dumped from your suit every time you raise your arm to take a photo or squeeze through a gap in a wreck.
A dump on your upper arm once set correctly take little user input allowing you to keep both hands free, they do take a bit more care to set you and use correctly but like anything a few gental dives to get things sorted and your on your way.
Go for a low profile auto dump, no more snagging on your gear as you kit up.
But at the end of the day, despite everything you read here, buy the kit that allows you to do the diving YOU want to do.
HTH
Darren