You're welcome.
I don't think there's much practical difference between the Greist or Singer zigzaggers. I think I've seen at least a couple people mention performance issues with them. I know at least one of them has interchangeable cams for different stitches.
Unless you just like playing with attachments you might be better off looking for an older ZZ machine to add to your herd. They're like potato chips. There's always room for one more. I know in my area I can pick up a complete working ZZ machine at a thrift store for $10 and up. A ZZ attachment from Ebay will cost about the same or more.
Rodney