I'd imagine having his ankle shattered in the early part of last season didn't help with Valencia's pursuit of individual honours. He still managed to recover from that horrific injury to put in some brilliant performances in the closing months of the season, displacing Nani from the team in the process.
The season before that he racked up
7 goals and 11 assists, as compared to Nani's 7 and 9. This was his first season as a Manchester United player. Which is usually reason enough to expect a much better return in subsequent seasons.
So far this season Valencia has been marginally more productive than Nani. Racking up a goal or assist every 117 minutes, versus every 129 minutes for Nani. He's done this despite spending a lot of the early part of the season playing as a defender and missing out on the whole of pre-season with an injury picked up on international duty. We've previously seen that players who miss pre-season can take months to get up to speed (e.g. Tevez and Ronaldo) Stats aside, I don't think anyone would deny that - taking the season as a whole - Valencia has simply been playing better than Nani.
At the end of the day, there's no denying Nani's individual brilliance. He's capable of doing things with a football that Valencia can only dream of. That said, any footballer is only as good as his end product and Valencia has blossomed into an incredibly potent and effective right winger. Improving season on season, despite an almost career-ending injury. If you imagine them as tennis players, Nani would be the mercurial genius pulling off outrageous drop shots and angled volleys but always capable of an unforced error. Valencia would be plugging away, running down every ball and hitting it back into the corners. That's a close game right there.
My own opinion is that, if forced to choose, I'd rather have Nani at the club. This is because I think he has the greater potential of the two. That said, not every footballer delivers on their potential and so long as Valencia keeps delivering the way he has this season I think people need to appreciate that this supposed gulf in quality between the two of them is nowhere near as vast as a lot of people on here seem to think.