I would go for the one piece.
The two piece while convenient can never be considered a real Dry Suit as it will not have a completely waterproff seal at the waist. They are more suited to someone who just wants protection from waves or spray - a sort of super oilskin . They are unlikely to keep you dry on immersion.
Ravenspring suits are fully breathable so you dont get all hot and sweaty or cold and clammy from wearing them all day.
Ravenspring also tailor make each suit for each individual. ( Do not be shy about your measurements!) We generally advise people to measurements over normal warm clothing i.e. what you will be wearing under the suit to keep you warm, Always round up measurements or as we say over here "add a drop for the cat !"
Don't forget that Dry suits keep you dry , not warm. It is the midlayer that you wear beneath the DS that keeps you warm and the base layer next to your skin like wickable viloft that keeps your skin comfortable. If wearing your own fleece jacket as a midlayer , beware of zips near your neck seal as they can compromise the integrity of the neck seal.
If you plan to use the suit for cruising or instructing then the optional fly zip is a must have extra !
Best wishes ,