Mollies are not appropriate for a 10 gallon tank though and would most likely create an issue with the betta.
I would vote for the otos. But in your case, I'd get only 2. Mine have no issues keeping my 16 gallon clean and they will also eat algae wafers, however, many people have written to say that their otos die because they refuse anything but natural algae.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~