Em's first two albums ARE legendary I don't care what anyone says and that nigga can spit.
Everybody he spits with on the same track he murders(see Em featuring on Jay-Z's The Blueprint if you don't believe me!). Only a few are able to raise their game and step up(Obie, Royce the 5,9 and Sticky Fingers come to mind),
But Nas for me is the better simply for longevity and he isn't as repetetive as Em.
Even though Nas is inconsistent, for some reason he still comes across (to me anyway) a proper M.C at all times.
Even his latest album, Streets Disciple which isn't the greatest of his albums for some reason as you listen to it and ignore the wack tracks it redeems itself, whereas Eminems last two efforts don't.
Just my opinion of course!

Eminem = all worthless ****.
Nas = all worthless ****. Excempt Illmatic.

I can see where you might be coming from. With such a great theory Eminem wins my vote.

this is my 2 favorites
there will no battle