I was underwhelmed with the 64 natural I had a while back. They're different cigars, so there is no direct comparison. However, it's worth noting that the natural version was not impressive while the maduro 64 is my favorite stick.
They are the same cigar; only the wrapper is different. The binder and filler for the blend is identical. Definitely try more than one of any cigar before you reserve judgement.
As for the Padron Millenium natural, if you like Padrons, you will like the Millenniums... whether the price is worth it or not is a personal decision, but the cigars do have a good 6 years of age on 'em.
Padrons are variations on a theme; it just happens to be a really, really good theme.