Dave, you are an excellent printer. Why not concentrate on whichever format is most comfortable for you and don't worry so much about the negative size. As Bernice said, 5x7 is an excellent compromise for both enlarging and contact printing. If you stick to 8x10 and need holders, I've got 3 wooden ones I'll give you. Just text me. 8x10 is typically not for the budget conscious. 5x7 is a much better compromise of cost vs image quality. Your 8x10 lenses will all cover 5x7 with room to spare!

Best,

Luis