Nintendo is running a promotion until the end of Jan that if you buy the Wii Fit meter and you download the game...once you sync the Wii Fit meter, you can keep the game free past the trial. But since you also need the balance board, is it better to buy the package or buy the Fit meter for $19.99 and finding a balance board at Gamestop for $14.99 used?
It's all up to you if you want to use a used Wii Balance Board. Considering how long it's been since Nintendo made new ones, there's the risk that used boards might be defective, might not. Also you've got to use up about 4GB of the limited hard drive space on the Wii U to download Wii Fit U, so that's another thing to consider. Currently Best Buy has this bundle (includes the new Balance Board) for $64.99...hopefully Amazon will price-match it soon.
only the Balance Board included with this bundle will work on the original Wii system and the two versions of Wii Fit made for the original Wii (Wii Fit and Wii Fit Plus). The Wii Fit U disc/game and Wii Fit U Meter that are also included in this bundle only work on Nintendo's new system, the Wii U.
Also, you said you bought the original already...I'm assuming you have a Wii Balance Board already. That Wii Balance Board is the exact same one that's included in this new bundle, so it can be used on the Wii U system for Wii Fit U and other Wii U games that support it later. If you want Wii Fit U on the Nintendo Wii U, you can buy Wii Fit U separately from this balance board bundle and then sync your existing balance board to the Wii U system.