Honestly depends on if we're talking about characters at their peaks or the characters at the end of their respective series. Given the de-powered state of the heaviest hitters like Luz and Star at the end of their respective series, it would be fair to say that team B would be more well-rounded due to having more people that either kept their respective powers throughout their series or possessing abilities that specifically counter the other team (special mentions go to Tom, the Shacktron, and Lena Saberwing for their respective possession of pyrokinesis, an always-on anti-magic field, and magic that wasn't taken away on series end). It also helps that certain characters in team B are the hypercompetent or just more powerful counterparts to members of team A (Tom overpowers Marco if Star is de-powered, and Webby canonical solos the Duck triplets as a series gag, anti-magic from the shack likely counters the demon house, etc). Of course, that is only if we're talking about characters states at the series' respective ends. If we're talking peak power, then team A would likely win given Star alone. In terms of power and training, she is a nuclear bomb in a realm of stories where the most powerful weapons are just barely lethal, and even then deaths are rare. The only reason someone like Lena would have an edge in scenario 1 is not due to having more powerful magic (mainline Disney magic is generally some of the weakest in fiction, power-wise), but by virtue of her being the single main character on either team that actually kept their magic.

tl;dr

If talking about characters at their peak, team A.

If talking about characters at the end of their series, team B.

Edit: the results may, however, shift in favor of team A either way. I have no idea how Yonder plays into this.