As far as pure amount of items in the store, Tannen's is a better shop than Fantasma. They carry a lot of products.
The bad about Tannen's is that there's a lot of pressure from the counter guys to purchase, the place is hard to find, and the hours are less than great. It's also kind of rare that the guys will actually demo anything for you.
Fantasma carries a bit less as far as pure quantity goes, but they do carry a lot of great stuff. They also have a huge display of authentic Houdini items, some of which are incredibly rare. It's easy to find (right next to Penn station), and is a terrific place to hang out. That, and the counter guys can and will demo pretty much ANYTHING. Also, you never know when you'll run into David Roth, Simon Lovell, Eric DeCamps, or any number of other magic celebs while you're in there.
I know this post sounds like I'm making Tannen's out to be evil. That's not the case at all. It is a good shop that carries lots of stuff, but it just doesn't have nearly as comfortable an atmosphere as Fantasma.
I definitely recommend stopping by both stores, but I think you'll be more likely to spend a larger chunk of time in Fantasma.
Also, you can check out Rogue's if you're in Queens at all.