IMO flat water SUP'ing will work your heart and lunges more and surfing will develop power and flexibility.. I reckon a bit of both is the way to go.
Laird Hamilton is more a surfing type body but the Chuck patterson does both and he's also built like the Hulk.
If you're coming back from a broken leg flat water paddling would be best but it wont hurt to hit the waves every now and then if the waves are small.
Check out this guy for low body fat and good muscle tone.. He looks like more your flat water paddler.
DJ