I usually use the same in both but a lot of people use a higher count in the bobbin.

I usually like to use 50 wt for piecing, top and bobbin. I know there are people who prefer 2 ply 60 wt. because they feel it makes a flatter seam. Personally, I am not convinced it is strong enough for a quilt that is used. I like to use 40 wt King Tut or Sulky for quilting and usually use Bottom Line in the bobbin. I have used Sulky 28 for quilting when I really want the quilting to show up. I am experimenting with some of the new polyester threads for quilting. I do not like to use mettalics because of the shredding problems. It is important to use the right needle for the task at hand. Superior Thread Company publishes great information. They have done a lot of research.