So, Wii Fit Trainer might be a wise choice from economical standpoint, despite you're still paying more than 3 times of the MSRP.
Functionality-wise, Kirby winds up being a slightly better choice, since he's compatible with more games than Wii Fit Trainer is. The latter is not exactly very useful, but she's certainly not one of the most useless amiibo either. Here's a chart that shows which game these amiibo are compatible and do anything special that you can't get with the other amiibo:
Compatibility chart
Game's name | Kirby | Wii Fit Trainer |
Super Smash Bros. for 3DS/Wii U | Kirby as figure fighter | Wii Fit Trainer as figure fighter |
Super Mario Maker | Unlocks the corresponding costume | Unlocks the corresponding costume |
Yoshi's Woolly World | Unlocks the corresponding Yoshi | Unlocks the corresponding Yoshi |
Kirby and the Rainbow Curse | Gives you the Infinite Star Dash for one level per day | Not compatible |
Mario Kart 8 | Unlocks Kirby amiibo costume | Not compatible |
One Piece: Super Grand Battle (Japan-only) | Give Kirby Costume powers to Tony Chopper | Give Wii Fit Trainer Costume powers to Nami |
Picross 3D | Unlocks Kirby picross | Not compatible |
New Style Boutique 2 | Gives you Kirby Purse | Gives you Wii Fit Trainer earring? |