Hi my opinion differs slightly from the above in that with a large model like the F18 the elevon is such a large surface even the smallest play from a quality servo like a JR 8411 will result in a fair bit at the tip of the elevon. (assuming this play IS from the servo). Providing the holding power of the servo is sufficient the likelihood of flutter is minimal.
I believe the best servo for this control currently is the JR 8611.

Hope this helps M