Banner Saga: Factions is the F2P game out now.
There will be a SP Banner Saga game (this summer, supposedly) later. When that is purchased, it will open up new unit-types for Factions. (That is, if you buy the SP versions, you can have different units.)
In the meantime, Factions itself will continue to evolve, with more levels, more unit types, more features, and (eventually) AI.