I have an idea for an F/m scene, but I am not sure whose viewpoint I want to take, which perspective seems more interesting?
Novelty is interesting. The domme already has a harem, adding one more sub to it is probably a relatively small change. Whereas the sub enters a whole new world: a new domme, new fellow subs, possibly a new home... It's a lot, and there's probably more things to explore from his perspective than the domme's. But it depends. If you have a bunch of ideas for how this will affect the domme more than the sub, then I'd say that's the way to go.
In terms of interesting the perspective of a sub could be the way to go. A domme's perspective seems to be more common so a sub's perspective is less explored and could have more to offer.