The thing is, a ZZ machine holds the fabric steady and moves the needle bar side to side to create the ZZ stitch.
A ZZ attachment moves the fabric side to side to create the ZZ stitch while the needle bar remains in the same place. The attachment is totally dependent on the vertical stroke of the needle bar. The shorter the stroke, the narrower the stitch. Also the attachment foot moving side to side makes it difficult to impossible to keep the stitch line straight.
So, go for the 401 if you need the ZZ capability.
Oh, light weight is sometimes a detriment. The lighter the machine, the more it will move around on you as your trying to sew.
Joe