Sasuke was trained by Orochimaru & fed for 2.5 years--but he still trained. As far as time, I say Sasuke. But the rules of Asura/Indra reincarnates shows that Sasuke has natural skill & power comes practically naturally if not handed to him (certainly not all, but still quite a bit).
Naruto ran around with Jyraia with his greatest feat being making his Rasengan bigger..... Wel, yeah that IS his greatest feat.
HOWEVER, he trained for quite some time & pushing himself to the limit in order to obtain the Rasenshuriken and again later on with Sage Mode all the while Kurama was hinduring him & trying to break out. Not to mention a lifetime of hatred & anger beating him with a stick & keeping to his word in order to tame a being of intense anger into perfect collaboration. All of this, like above, follows the rule of Asura/Indra reincarnates in that Naruto was a flop who had to undergo hard work, teamwork & eventual leadership in order to obtain power--and every time he did, he made a massive leap forward, so as far as effort, I say Naruto.
So, my opinion is quantity of time goes to Sasuke, work goes to Naruto but all around as far as work and power they're once again tied.