The Shield.
I've had both, and I really like both, and the Ruger is a little more concealable; it'll fit in most pants/shorts front pockets. Shield isn't quite that small.
But the Shield is more reliable, there's a little more to hang on to, so easier to manipulate, more shoot-able, better sights, better trigger (not by much), larger capacity.
Like I said though, I've had both, and actually looking to get another LC9S pro. Sometimes it's nice to be able to just drop a little gun into your pocket and go.