If you are just starting to FMQ, I do suggest you use the 50 wt thread so that you can see your quilting. But regarding what weight of thread to use actually depends on the end result that you desire. If you want the quilting to be like a shadow of design then you would use a 60 or 80 weight thread. If you are wanting the quilting to show then you would use heavier threads like 50, 30 or 12 weight. Always keep in mind your actual design, if it has some backtracking in it then you would want to use a light weight so the backtracking doesn't show so much.