I voted Mateo for 2 reasons:
1) He played 25 games last season and looks more durable (although SBW DID manage 21)
2) Salary - you've got a helluva lot more salary cap to play with if you have Mateo right now, you can sign up a 250k per year player to go with him, and that probably improves your team more than 1 SBW does.
On this years form, Mateo has been stronger.
1.5 linebreaks per game compared to 0.5
4 linebreak assists to 2
3 try assists to 0
He runs the ball 15 times per game compared to 12
17 tackles breaks as opposed to just 4
10 offloads to 5
2 missed tackles per game compared to 4.5
Marginally less errors (1.7 - 2)
The tackles are about the same (20 -19 in favour of Mateo)
Mateo has even kicked the ball in general play!
The only area that SBW is currently ahead is meters per game, but it's marginal! (136 to 127)
It's not an indication that Mateo impacts a game more, but only a fool would argue that stats are no indicator of performance! If you looked at the numbers of the 2 players last year, they were pretty similar - and SO FAR this year, Mateo's have spiked as has his comfort level in first grade.
Mateo - less injury scares, cheaper, freakish talent that he's putting to good work.