The topic actually started as powerhead for a 20G tank, so basically Koralia 3-4, and all the Seios are out of this category. I think the smallest Seios were designed for at least a 50G tank, but I'm not sure about that.

Koralia nano, 1, and 2 certainly smaller than any of the MJ's. Much more easier to hide them in the aquarium. If you buy a new MJ 400 you spend somewhere around 20.00 bucks + the conversion kit another 15-20.00 dollars, or you can buy a K nano for 25-30.00 bucks.
I'm on a budget, I'm sorry.

The other real benefit of the Koralia is the lower wattage, so the water will stay cooler. You might have a chiller, than you don't have to worry about that part.

Most equipments in this hobby are still using suction cups, so the magnets aren't the reason I'm buying these powereheads. I was looking for a small wide outflow powerhead for a nano tank, and I'm glad I found it. Magnets are a plus for me.

Besides all that, Maxi Jets are great power heads, but I found the Koralias more efficient when it comes to water circulation ,specially in smaller tanks.

Uhmm...whatever