There is a certain risk for both ways and both have their pros and cons. Just counting which one produces more goals is not possible as it would ignore the fact of many shots being saved, blocked or going wide. Most corner kicks get headed out of the box by the defending team, regardless of the type of spin. Also having a player like Lewandowski or Ronaldo converting anything they touch it would not matter how you pass them the ball.
But why do you spin it in or out? This primarily depends on who is playing and the game plan.
Spin towards the goal:
- easier to handle for the goalie
- good opportunity to come closer to the goal
- good for a cross to the far side of the box to have another player pass it back to the middle, so someone else can score from there
Spin away from the goal:
- goalie has a harder time to catch it, because of a longer way getting to the ball through the crowded box
- if the ball gets too close to the goal line it will eventually pass it in mid air, so it's not a good way to get close to the goal
- you can target the whole box, so it's a good way for direct goal scoring opportunities
And which one is now better?
None, it's the mix of both. Alternating styles and player positioning is more important. Obviously it will be harder to defend, when the defenders don't know where the ball goes.
And what about the stats?
Many teams have two or three players doing all the corner kicks. When they're all right-footed they don't care about the spin, because that's already decided by the side they're kicking from. This would render any stats useless just by the nature of the game. Also it heavily depends on the players in the box being able to convert the corner kicks.