Also, are they well-behaved dogs?
No real difference: The breeders offering "miniature", "toy", and "teacup" Pomeranians are all unethical and/or ignorant twits, and you don't want to buy from them.
That's about it.
Honestly, the Pomeranian is a Toy breed, as in a member of the Toy Group. It's already small — anyone claiming to breed "mini/toy/teacup" versions is a scam artist breeding runts to runts.
There is only ONE Pomeranian.
http://www.americanpomeranianclub.org/about_poms.htmTags: Pomeranians, Teacup Pomeranian, Pomeranian