I use 50/3 for both. I am thinking about moving to a 60 weight for the bobbin for piecing. You want to stay away from thick threads for piecing as the thickness of the thread makes a difference in the final block. It also distorts the pressing.
If you want your FMQ to blend, use a 50 weight in a color to match. If you want the stitches to show use either a contrasting color or a thicker thread.
A lot depends on the machine too. Some are persnickety. Mine WILL NOT use Coats and Clark dual duty but it likes Coats and Clark Star brand. Go figure.