I have two of the sixes and two of the threes. I love these systems, I have my bettas in the 6g ( 1
betta=1 6g ( with otos). the bio filters are really good, I like the neat look of them, with all of the stuff in the hood. I also like the bow front, really great viewing without the corners with the silicone in them like the glass tanks. The filters are efficient and quiet, easy to change the cartridges too. I have seen some complain of the water flow from the bio wheel with a betta, but personally, my bettas like to play in the outflow!!
I don't think you can go wrong with them!
Good luck!
Val