I have both these grinders, so regularly I am comparing them, across a variety of different machines, brewing methods and different beans.
The K3 certainly does give a more consistent, less clumpy grind when grinding really fine for espresso. The flavour profile is richer and deeper. It takes longer to blond during shot.
Smart grinder is better for filter or cold brewing, but also holds its own with some types of beans for espresso, sometimes i like it when using one of my levers, which seem to give rounder, deeper flavours straight off the bat, using the K3 ends up rich little choc hits, depends what you like or feel like at the time.
Overall if had to choose between them K3 wins, but I still think the SG represent excellent value and quite user friendly.