Oh I am so possibly going to get stomped on for this... (keep in mind, fish keeper 20 years, breeder of different species throughout the years, etc.. so this isn't my first rodeo :) )
I actually use these to house some of my pet bettas/breeders in and I have no issues with this tank.
I divide the tanks into three sections. Yes! 3 bettas in the tank! Oh my!
There is actually more room in there than you think, it's like a perfect square for each one that gives them the room to swim around in and be happy. I'll try to get a picture of one set up soon.. but it gives them roughly 2 gallons each, which is a fair amount for a betta. You can have a happy, healthy betta in 2 gallons, they do not need 5 gallons. These are not wild bettas who keep 3 feet of territory.
So I say, if you want to divide it in half, or in thirds, go right ahead as there is still plenty of room for them. It's about water quality, not quantity.
As for the filter that comes with it.. if it's loud then turn it down.. I put it close to the lowest setting (so there wouldn't be any current) and I hear nothing. I'm sure it's loud if you have the filter turned way up though..