Muay Thai is more effective. Since it allows for more techniques than kick boxing PLUS all the techniques from kickboxing it's logically more effective or equally effective.
Also, Cro Cop is a Muay Thai fighter in the aspect that he uses low kicks and clinch + knees. It's just that K1 doesn't allow elbows, and only one knee from the clinch and then you have to let go, so it's not very Thai-ish.
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