Impossible to say, because they function differently, and there's NMs that a SMN is more favourable against it, and theres NMs that a BST can solo better.
inb4 biased SMN and BST trying to white knight their job, i'm a main SMN and it's my baby, but knowing your jobs strengths as well as its weaknesses is just as important as knowing how to gear and knowing the jobs mechanics.
One of the main differences in SMN and BST is how they deal damage, SMN can give out massive spike damage, but their DoT from avatars meleeing is rather poor. BST gives out impressive DoT from pets but their spike damage isn't as good.
BST pets have a lot more HP than avatars, paired with the ability to use reward to heal them can give a BST the edge over SMN when soloing because they can use fewer pets, and the hate control is much better (especially with snarl). however if their pet dies before call beast is up, and can't charm anything nearby it gets dangerous.
SMN pets have roughly the same HP as a player but they have a native -50% physical damage taken, as well as a stout servant trait around lv80~. They take full magic damage but they heavily resist their own element and the element they are strong against, making them much better "tanks" against mobs that use a specific magic damage type.
SMN has an unlimited supply of pets as long as they are able to summon them and they have the MP, which makes them more favourable over BST against things that can down pets quickly.
Also if you were to try argue which one is the best, you would have to compare a skilled SMN and BST with equal standard of gear. A crappy BST won't be able to solo as good as an amazing SMN, and vice-versa.