A 494:

Dismissal couldn't be used against a swarm, because (from the swarm traits)
A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate)
, and Dismissal targets a single creature. Banishment, however, can target any number of creatures, so long as they're close together, and so this restriction does not apply. It does have a maximum HD for its target, but the swarm traits don't care about that (and any swarm is likely to be under that limit).