First of all, I have to admit I became a Pep fanboi over the years...
Now back to the topic: To me Alves is a perfect example of what Pep can do to players, or rather what he can do with the player material he got.
Under Pep in Barcelona Alves was for several years THE prototypical perfect fullback. I still cannot see anyone playing that specific role any better. But, and that's a big but, in Pep's system a fullback is very rarely challenged defensively. That's simply the result of his system, and the reason why Rafinha, a pretty modest defender himself, nowadays looks like a great fullback at Bayern. And that's also the reason why Pep put Lahm into midfield. Pep believes that Lahm is wasted at FB with his qualities. And he’s right since Lahm has much more to his game than just running up and down one side.
Nevertheless, Alves at his heyday in Barca, only going forward with all his abilities, he was next to Messi and Xavi the most important player of that awesome team. Hence, I would give him my vote for looking better at his peak.
Still, I have voted for Lahm. Simply because he is the better overall football player, the more complete fullback, the better football brain. If we go back 8 years in time, and if I had to pick a player out of the two to start a new team, I would pick Lahm without thinking. Simply because he is better. He is probably the best FB in history. That’s why I voted for him.