That was about the difference in quality between my 2 GK's at that point.
For me the turning point was a defeat in the League following a poor performance by him in the second leg of Cup's Round of 32.
Their stats to further prove my point (incl. Association matches)
Radyukevich: 20 matches played - conceded 17 goals. Average rating of 6.99 this season, 0 times MotM.
Zboncak: 19 matches played - conceded 9 goals. Average rating of 7.4 this season, 1 time MotM.
I believe it's a combination of both quality and form, but form is the most indicative thing about who you should pick to start from my experience this season.
I also don't think that a 10% difference in quality makes a lot of difference anyway. I will always pick the better performer.