IDK- my kids had periods of times when they wouldn't grow much & wore the same outfits several (yes several) years in a row. My DDs were a devil & angel for 3 years because the costumes were big enough! lol
Personally at 18 months is 1½ years old, and more than old enough to have something they like. If you can afford the 20 dollars to buy the costume that your daughter would like that's what I would do.
The only way I would use the care bear one is if I couldn't afford or do without something myself so that I could afford to buy what my child wanted. I don't personally like yo gabba gabba, however I would still make my child happy. And that show is allowed in my house. Unlike barney which will never be on in my home. Period.
Id do the care bear. My ds is 16 months and if i dressed him in a costume i knew he'd love, i dont know if he'd ever want to take it off. Instead of buying the $25 one he'd love, i bought the $7 one. Like you said, they wont remember it anyway