I think that Mack looked good in the GCO partly due to Flowers and having some large holes and good speed. But, don't sell Mack short... his first season here he was in a more pro-style offense and honestly, his draft stock may have been even higher if CWT had stuck with that. From his first game, you could tell that there was an extra level of vision and patience that he possessed to allow things to open up.
Cronk probably has all of the skills, but, I don't know if he has the vision that Mack had. Forget speed, vision is something that RBs at the next level need to be successful. Frank Gore at 35 years old had a heck of a season for Miami this year because he has elite vision, probably as good as anyone who has ever played... He certainly doesn't have 4.4. or 4.5 speed (I don't think he ever did), but, a guy who can get you 5 ypc in the NFL is more valuable than a guy who can break a big one every 200 carries or so. As a dolphins fan, following the reporters, it was fun seeing how they all marveled at how Gore looked in practice and how different he is than the guys on the team that probably have more physical talent at this point.
I think Mack is a good combo of speed a vision. He has homerun ability, but, decent enough vision to wait for a hole to open up and get what is there on most plays too. His size may limit him at some point as the speed fades because he does struggle to break tackles on negative plays to minimize the damage.