They're pretty similar, in fact, they're almost identical. Almost. Most Cetmes won't accept G3 mags, but some will. Mine won't.
I would avoid the Century-assembled Cetmes and G3s if I were you. I don't know about the more recent ones, but all of them that I've seen have been junk. Cocking tubes welded on crooked, mags (Cetme or G3) that don't fit at all, and misfeeds every other shot.
It's possible to rework the magwells to make mags fit and this usually cures the feeding probems. I've reworked my Cetme receiver to fit Cetme mags and I tihnk I've gotten it to feed, but I heven't shot it yet.
Century can barely build an AK, there's no way they could build the Cetme/G3 receiver. They're simple, but it's sure hard to stamp and weld a part to the tolerances required.
Century builds G3s now, I think they ran out of Cetme parts. You should check their website. JLD builds G3s of much greater quality, but they're expensive.
If you're thinking about going to a gun show or something and picking up an inexpensive used Cetme, just make sure the mags fit well and maybe bring a few snap caps or dummy rounds or something and try it out before you lay out the cash.