I recently got sponge filters for my 2.5 and took them out soon after. The size and type took up to much space. I will just stick to my water changes for now. Mine were pretty standard circle bottom sponges, but I do know there are some that stay higher up in the tank and take up a little less space. After thinking about the benefits yes they are good have but I will stick to water changes to give my boys the most room possible.
Hopefully someone can recommend some specific sponges that will be better space saving.
You can also browse around online for DIY sponge filters so you can make your own, especially if you're on a budget. If you're interested in them, especially making your own, I'd recommend looking into good air pumps- I made one myself but the cheap air pump I had was VERY... very.. noisy >.< Other than that, the only reason why I didn't keep it is because I wanted to get plants and with the plant substrate being added, the dragon air pump I had in there would've been too tall... Also, keep in mind to get some sort of air control valve or something to let out any excess pressure- you'll be able to control the flow of the air (on the off-chance your betta is kinda picky about that sort of thing) and you won't have added pressure trying to build up on your pump.
If you really don't like the whole nuclear reactor look of a sponge filter you can hide them behind some deco or plants, but their look doesn't bother me. My fish actually like to sleep under or on top of the sponge, and the sponges hold beneficial bacteria.
As Draug said above, you can also find a lot of info on how to make your own sponge filter. If you dig through my old posts I have a DIY post with links to a lot of DIY fish projects I found on different sites on the internet.
There are some other sponge filters out there made my different companies but I went with Hydro after reading a lot of good reviews on them. I would think most of the sponge filters available would work pretty much the same.
Even with a sponge filter you still need to do regular water changes, and keep the rocks siphoned. (for whatever size tank you own)