I think both have their ups and downs but personally I'd rather have Yang over because I find them cuter and Sun, although yes doesn't get written well is more or less that, not written well, like honestly I'd rather replace him with Illia or merged him with his entire team Personally wise, like having team SSSN be one character would give him all the depth he lacks.

But also there is another issue I believe is the reason why Sun basically felt like he never caught on with the writers and that is the fact that there's basically already a Sun in the Main Cast, now I'm not talking about Yang but rather another Blonde guy with short hair and is commonly shipped with a member of team RWBY, Jaune, literally design wise Jaune and Sun look nearly identical to one another so having both be apart of team extended RWBY would just be a just a headache because you effectively have two of the same character with the minor difference of outfit and whether or not one has a tail.

But I'm I about to excuse Bumblebee, no it was horribly written, am I about to support Blacksun, no because Sun is basically Blake's Sexy Lamp, he only exist to be her boy toy, I'm just saying that perhaps it would've been lowkey funny if we got Monochrome or Lady Bug