First thing, welcome to Tropical Fish Keeping forum. Glad to have you with us.
The smaller filter will be more than adequate. At this point, I will mention live plants. If you have them, they will do most of the filtering anyway, so a simple sponge-type filter (i.e., the foam pad) will be sufficient, with as low a flow as possible. If you do not have live plants, I strongly recommend them. Not only for their filtration, and aesthetic beauty, but Betta occur in plant-thick quiet waters and they expect to spend their time browsing among plants at the surface.
I will also mention our profiles, under the second tab from the left in the blue bar across the top of the page. If the scientific or common name used in the profile for a fish is used in a post, it will shade and you can click it for more information on that species. Example, Betta splendens or Siamese Fighting Fish.