Half the player is pushing it to the extreme. Valencia was much better at ball retention, pass completion rate and was Mr. Dependable while still being productive in a possession based United team. They had a fairly even number of assists up until Valencia became a right back. Nani was much more explosive though and his ceiling was a lot higher, but his lows were also lower to the point where he'd be a detriment to the team at times (mindlessly losing the ball against Chelsea in the semis in the last minute of the game from which they go on to score and knock us out). It's sexy flair and explosiveness vs. boring dependability and consistency so it's easy to look back on Nani more favourably.
I'd probably take Nani if given the choice, depending on the context of the game at hand. I don't think Nani ever maintained his peak long enough to qualify him as 'world class', though.