I like the idea of trying out different needles on quilt sandwiches. My best guess based on experience is that 80/12 microtex would work well. However, a lot depends on you FMQ technique. If you quilt fast you might need to move up to a 90/14 microtex to prevent the needle from flexing as you quilt. I prefer microtex because of the smaller holes and sharper point. I will switch to topstitch if I am having trouble with the thread. Aurifil 50 never gives me trouble, though, which is why I would choose microtex. In general, I want to use the smallest needle size for the thread that does not cause the needle to flex. If you FMQ with foot pedal to the floor, then you might need to go up a size because you will be moving the quilt faster which in turn puts more stress on the needle between stitches.