The difference may be in the way that the two telescopes are constructed. Tube currents can be the major part of the problems with a scope. Using a little fan to blow the air out of the F6 scope may improve its performance a lot.
As to testing a scope, you may want to make up an artifical star so that you can test in the daytime. Almost any nice reflector ball (Christmas ornaments are always mentioned here) will produce a nice bright spot where the sun shines off of the surface located far enough away (there is a formula for this on the web for spacing artifical star sources from the scope) will make the job of star testing a lot easier. Generally the formula produces a minimum distance at which the aberration from the telescope being used in a near distance view will be approximately 1/4 wave spherical error with 2x the distance producing 1/8th wave error and so forth.