I've begun to think of "wemics" and "liontaurs" as two different things. Wemics are a Dungeons and Dragons monster, and they are not included in the Standard Reference Document created by Wizards of the Coast as a kind of open source version of the third edition rules. (Several good SRD links are on the left-hand column of the home page.)
That means Wizards may choose to claim "wemic" as a piece of intellectual property, like illithids and yuan-ti. I live in fear (if I may exaggerate) of getting a cease and desist letter from the company telling me that I can't have a "wemic" site -- and that's also the biggest reason I've never moved to a "wemic.com" domain.
Take a look, for example, at the errata and clarifications for Monte Cook's "If Thoughts Could Kill". It says, "Throughout the book, all references to "illithid" should refer instead to "mind flayer." Illithid is a protected term owned by Wizards of the Coast and is not actually in the System Reference Document. Silly us."
But the generic term, "liontaur," does not have that disadvantage, I think. I suppose the Sierra Software people might claim they created the term for their Quest for Glory computer game, but I'm not so sure that's true. And besides, as I found out a few weeks ago, the lion-centaur has a mythological origin dating back thousands of years! No one living today can lay claim to owning the idea of a liontaur.
But aside from the idea of generic name vs. brand name, is there a difference? Well, for me, I am tending toward making this game distinction:
Wemics are formidible, beastial, and huge (large in size); they have five racial hit dice. They look more like lions, and might stand as tall as seven or eight feet. Think bulging thews here. A wemic character either starts with five monster hit dice or perhaps starts as a young cub who grows into his racial dice as he gains experience and gets older.
Liontaurs are human-like and big (still large in size). They are basically human from the waist up, aside perhaps from cat-like eyes and sharper teeth. They might stand as tall as six and a half feet, but many, especially females, might not grow above five feet, six inches. As characters, they start at first level with a class -- no racial hit dice.
Use "wemics" in your game if you want fearful and powerful creatures who can stand toe to toe with ogres and win. Use "liontaurs" if you want a human-equivalent cat taur race in your game that you can use for player characters from level one -- and not worry about unbalancing the party.