Yep. Smallest Whisper or Aquaclear. Bettas like as little movement as possible. In my betta tanks I use a lot less GPH than other tanks. I do more water changes to compensate.
I'd also put some floss or a coarse sponge over the intake just to 100% ensure that the betta doesn't get injured from coming too close.
bettas could care less about the flow, just might take some getting used to after sitting in a bowl for a good while. My 5 girls love to play and "fight" the current from my filter, lol, its quite amusing sometimes.
Oh, and I suggest the Tom Rapids mini-canister filter, any canister filter is da bomb IMO