Cottage Dee

I love aurifil...but I'm a little confused...when do you use 40 wt or 50 wt? I see aurifil Mako 50/2 on the one in my sewing machine, but I also have Select 40/2 which looks like it is stronger or even might have a coating on it. Currently I am piecing an Irish Chain with the 50/2....should I be using the Select spool? I also hand piece, but use the 50/2 for that. I'm the kind that just picks up a spool of thread and uses it in my machine. However, I just had my baby cleaned and oiled, etc, for a pretty penny (I bought my Pfaff in 1986) and I want to keep thread fuzz down ... So what size should I be using to machine piece? I'm needing to buy some more thread and want to get a good deal....my lqs sells the 50/2 for $10 each, plus tax, of course!