When I make the Bow Tucks bag I use the WM heavy fleece just for the outside bag. The lining, outside pocket, inside pockets and handle are done using the thinner fleece. I tried to make one useing all heavy fleece but was way to stiff and hard to sew especially the handle. I like the bags a little firmer so decided to just use the heavy fleece for the outside. Worked out great.
HTH