Last season
Fouss DBPM 3.7
Keita DBPM 4.0
Fouss DRtg 97.5
Keita DRtg 94.8
I just picked the first two advanced stats that came to mind. I'm not posting the other players' numbers which would make it clear how close these numbers are in both cases, but you can verify if you want. For example, Fouss' DRtg is better than anyone else on Utah's roster except Keita.
While Fouss has a big advantage on offense and experience, FTs, and foul rate, Keita has a very slim advantage on defense. Last year Keita rebounded a bit better but their career averages are practically the same there.
I think who starts may ultimately be matchup-dependent through the year, but if it's based on ability and accomplishments Fouss is clearly your game 1 guy.