I free motion quilt on all my machines and they all do a great job. Until I bought the Sweet16 I used my Brother PQ1500s for big projects unless I wanted to FMQ using a zigzag stitch in which case I would and still do use either my Brother PC420 or my Brother VX560. Now I mainly keep the PQ1500s set up for piecing since I find I can be very accurate with it. Most of the time I use the Sweet16 for FMQ since it has extra big bobbins and a big throat with lots of visibility all around the needle. I did recently use the PC420 to fmq a large project because the 100wt thread I was using kept shredding (caused by being too heavy handed with the spray baste). I have anti-glue needles (which helped some) for the PC420 and it has an auto needle threader which makes life a lot easier when you have to keep rethreading.
Ergot- I use whichever machine will get the job done the easiest.
Rob