I think there was a reason Green Bay selected another running back a round or two after taking Lacy. I loved watching Lacy run the ball, even if his stats weren't mind blowing, seemed like he was doing something special every time he touched the rock. However, the whispers were that his injuries scared off a lot of teams and were the reason Green Bay selected another RB shortl after taking Lacy as a hedge against his injuries.
But I'm not sure that even matters. I think Gio fits what THIS Bengals team needs more than Lacy. Gio has an extra gear that Lacy doesn't have. He wont break tackles and pick up the tough yards as easily (though he's far better than I would have thought) but BJGE is great in short yardage situations, always grinding his feet picking up an extra yard or two after initial contact. Gio is better out of the backfield in the recieving department, he gives this offense another dimension that Lacy wouldn't have brought to the table. On that TD reception, Lacy probably gets shoved out of bounce by the safety, Gio ran by the safety (and linebacker) for an untouched TD.
I don't think it matters much who has the better overall career, its a matter of who fit the Bengals offensive holes over the next 2-4 seasons.