Essentially, both "Fresh Prince of Bel Air" and "I Want to Break Free" explore themes that can be linked to those highlighted in Billy Elliot, essentially allowing you to use either one of them as a related text for Billy Elliot. In terms of choosing between "Fresh Prince of Bel Air" and "I Want to Break Free", "Fresh Prince of Bel Air" seems to explore themes that are most closely linked to those of Billy Elliot, which could make it more suitable to be used as a related text. On the other hand, "I Want to Break Free" seems to be less complex and this could be reflected in its themes. When compared to "Fresh Prince of Bel Air", it is unclear whether it would be a good option.
Additionally, I think that you should consider the type of text that you intend to use as related material. Since it seems that both options are songs, there is a potential concern that neither of them would offer sufficient information (given the limits of duration and lyrics) that enables you to engage in a more in-depth analysis. Based on this, you could consider other types of texts that can potentially offer more avenues for in-depth analysis, such as novels and poems.
I hope this helps!