Luck is about as sure a thing as you can get.
Of course, there are no guarantees, but it seems safe to say that with Luck, the worst any prospective organization is going to get is a career starter at quarterback.
The best it will get is a Hall of Famer.
Robert Griffin III
Some argue that the direction of NFL signal-callers is changing. It is moving towards more dual-threat quarterbacks ala Cam Newton or Tim Tebow.
In this capacity, Robert Griffin III is ideal.
As previously mentioned, he needs to put on some muscle mass to absorb more NFL-level punishment, but in 2011, there was no more dynamic player in all of college football. If he can bring that dynamism to the NFL, he will be a force.
Luck is a great quarterback and more of a sure thing than Griffin, but he is physically unable to do some of the things that Griffin can do; moreover, no amount of experience or development will change that.
On the other hand, Griffin, with more development and experience working in a pro system, could arguably learn to be as able a passer and signal-caller as Luck.