FM gives lots of health, a little AD for those many auto attacks and a great slow.
Rylai's gives a good amount of health, a good amount of AP and a decent slow on the Blind and more slow for the Shrooms.

I haven't build either on AP Teemo yet, and am curious as to which would be better? They both seem fairly good.

Edit: Clarified the Rylai's benefits.

Just delete Teemo. ;_;

i dont think ry applys on your auto-attack

so if it infact doesnt and you are looking for a slow over more poison damage, go for fm, if you want no slow (assuming it doesnt work on the auto-attack ability) but higher poison damage then go for ry

None of the above?

Rylais doesn't apply on your auto.

Why the downvote? I know I'm right. Brb finding proof.

I think Rylais procs a lesser slow on the dot, but I could be wrong

FM works better early-mid game, where you can actually auto attack and not explode.

Eventually, though, there will be a point where it's dangerous to be even near auto attack range, in which case rylais help more due to shrooms and q.

Also, rylais doesn't apply on poison, since it's an on-hit effect.

Rylais doesn't apply slow on auto. Frozen mallet does. And it gives more hp.No escape from fro mallet teemo.

Toxic Shot does not proc spell vamp and Rylai's slow

http://leagueoflegends.wikia.com/wik...bility_Details

FM is always better. AP Teemo's ratios are stupid high at the moment and you can afford to have a more powerful and reliable slow on a slightly tankier item over 80 more AP.