Which should I get?

[Gunner]

I am looking at gettin either an 18" or 12" machete from Ontario Knife Company. My question is whic should I get? If I get the 18" one I would get the self sharpening sheath for it, is this sheath any good? Does it sharpen the machete well? And if get the 12" I would have to get the nylon one. How well does it hold up? Is there an alternative to nylon? Do they offer any other type sheath for the 12"? Thanks.