Muntari was a disaster last season, yes. But he still had a good 1st season with us. Biggest problem with Muntari is his erratic passing and his headless tackling. Kharja's tackling isn't that much better either tbh. Muntari is also much stronger on, and off the ball, and is actually able to make nice sliding tackle's at times, and win the ball with a shoulder-shoulder challenge.
Muntari also has better positioning, and is much better in the CM position. Due to his stamina, he can close more gaps than Kharja and he's actually pretty good at marking out speedy wingers ... unlike Kharja, who leaves major gaps everywhere. Kharja's best position is ACM, a position which he also doesn't really offer anything extraordinary really.
I also recall Muntari making some nice through passes, and has a good long range shot. Kharja mostly only passes sideways.
I would rather have no one...but if I had to choose I'd pick Muntari.