He's a better #10 in the sense he's more natural at it. He'll get similar number of assists as Rooney but Rooney will look better at it because of the goals he scores. Rooney scores more because he's really a #9.5 or at the very least a more natural #9. Stats don't tell the whole story though.
When Rooney plays as a #10, he plays too close to the centre forward so we end up with a massive gap between the forward and midfield lines which other teams exploit. Wayne also doesn't have the nous to thread the ball through the lines or even direct play anywhere near the centre, instead all he does is spread the ball out to Valencia (practically every time he gets the ball too). Now this is great if we want to rack up the corners but not for actually scoring goals because a) it's a predictable tactic and b) Valencia manages to hit the knees/shins of the first defender with 100% of his crosses. You notice as well that Rooney and RVP just don't fit together, they barely ever pass to each other and it's only on rare occasions that they create a goal for each other. Granted when they do it's usually pretty good (away at Cluj, home against Villa for example) but it's so rare and at the top level you need your forwards functioning together.
If we put Mata or Kagawa there, we have fluid movers who change up their style of play to suit the moment. They'll play little triangles, pass and move, thread little through balls in between defenders and so on so we immediately become much more fluid and dynamic in our attacking play. Rooney is a complete detriment to us as a #10 regardless of his individual numbers and I'm sick of seeing the same tired, stone age play that we create when he's put there. We need to move on because he sticks out like a sore thumb.
Have him and RVP fight it out for #9 and have players in their proper positions behind creating the chances. If that means Rooney is gonna get benched then so be it.
Anyway, back on topic. Sanchez is an extremely dynamic player who can play all across the front line. If we got him then we'd have lots of interchanging much like the Ronaldo era (and yeah, I'm aware Rooney was one of the biggest influences in that period) and we'd be very unpredictable which is great for scoring goals.
