If you're upgrading to a 30 gallon tank then you don't want to slow the flow rate of a filter at all. If you do you'll have dead spots in the water
. You don't want any stagnant water in your tank. Unfortunately this is an issue when you begin dealing with larger tanks and betta because those larger tanks need that extra water movement. After having gone through too many filters for my 30 gallon tank, I ended up with an AquaTop CF400-UV Canister, and have never looked back.
For a 10 gallon tank I highly recommend the Rapids Mini Canister. It comes with this cool plastic piece that wraps around the canister allowing you to hang it from the back of the tank. I have several of these canisters and they're seriously awesome and very very quiet.
Looking at the photo you can see the ceramic rings on the right side of the canister. If you only add half of those, you can add a bag of Purigen on top of them, and on the left side you can add a drum shaped sponge (they generally come with a hole in the center, so it will slide over the tube.. you can't visually see it in the photo because of the charcoal bag in the way. Those soft charcoal bags that come with it are terrible).
You can use the same drum shaped sponge to slide over the spraybar, which will slow the flow rate and make it comfortable for a betta. What I've also done is ensure the spraybar is under the waters surface, and make sure the water holes are pointed straight up at the ceiling. This provides a lot of surface agitation for better oxygenation in the water column without blowing the fish all over the tank.
Here's a sponge pack, so you would have enough for inside the canister and for the spraybar.