I would play the one who's in best form, if they're more or less as good as each other i would rotate every now and then. Right now i think it would have a negative effect if Mitrovic gets dropped at first chance after waiting patiently for ages for his time to come, and when he's now got it and grabbed it by scoring 4 goals in 2 games he should continue at least the next game which is tomorrow to get the signal he isn't gonna be dropped every time Gayle is fit. It would be too harsh too soon.