40mm micro or extension tubes?

Hey everyone.

I really want to get into macro photography. The only two lenses I currently own for my D90 are the 35mm1.8 and the 85mm1.8D. My initial plan was to go for a 105mm macro so that i woundn't be too redundant in mid range telephoto, but all the options are too much for what i'm looking to spend. The 40mm micro is pretty cheap used ($250 refurbished) but that's almost identical in focal length to the 35mm. The 60mm is also manageable if I get a good find, but again, it just seems to close to the 85mm to be worth it.

So my question is, is it worth the 200 bucks for the kenko extension tubes to use with my two current lenses, or will I be more satisfied just buying the 40mm micro?

Thanks in advance.