There are three different questions:
1) what is a player's upside
2) what are the odds a player approaches that upside
3) what is the players' most likely outcome
To me, Myers is "safer" than Sanheim, because if he doesn't blossom offensively, he could still had a solid career as a Coburn type, whereas if Sanheim never develops, he's more likely to be someone like MDZ.
Myers also has a higher ceiling as a potential two way force, whereas Sanheim right now looks like your typical offensive defenseman who you hope is adequate on defense.
They both have much higher ceilings than Morin, who lacks the offensive skills to be a force, and still has to show he can develop into a shutdown defensive defenseman.