Unless you have a very mediocre mirror the 10" will usually outperform the 6", given that the seeing conditions permit. I have a 10" f/6.8 and a 12.5" f/5. Also recently had a 6" f/10.5. Most of the time the 10" is the superior instrument. It has an excellent figure, the 12.5 is slightly undercorrected. The 6" was spherical but gave outstanding images. I did find that the images were sometimes too bright in the 10", but various filters were advantagous to bringing out further details and cut down on intensity at the same time. Do a star test to find out what your optics are like and go from there.