The smaller individual jar.
Third best, but small cups are not included. Would YOU like to live in a closet? I think not. So why should your betta be any different! For heaven sake, spend $5 and get him a decent size jar!! OK. If you have a bunch of bettas, and not a whole lot of space, you might need to make those jars smaller. I do not recommend anything less than 1/2 gallon. 1/4 can be used but only as a temporary housing (say you just jarred a bunch of young juvenile males and are taking them to the pet store in 3 weeks or something) (by the way pet stores are often TERRIBLE with their pets, so I never take my bettas to a store, ever.) Get the idea? With 1/4 jars you need to do a full water change every third day, if you don't want those bettas to get tail rot, and other bacterial diseases. With a 1/2 gal jar, a weekly water change should suffice, although twice a week is still best. I keep all my jars bare, because it is cleaner and easier to make the water changes. Remember I have hundreds of them. If you can add a live plant, do. Of course the smaller jars mean more pollution so beware to not overfeed!! Uneaten food rots promptly and turns into betta disease pronto. Also, remember that water change remain stressful for the bettas so avoid doing them daily, because you will probably weaken your betta's immune system that way. A good balance between too much handling and not enough water changed must be achieved. Again REMEMBER TO COVER ALL JARS AT ALL TIME. A must.

I copied this from a web site.