The difference lies in what's inside.
The aquaballs have cylindrical sponges with a hole down the centre in every basket, and a small box at the top which eheim recommend you fill with substrat pro. All the baskets have slits in the sides. Water is drawn into every basket, through the sponges to a central core, then up through the top box and back out.
With the biopower filters, only the bottom basket has a sponge, the same type as the aquaballs. The other baskets are filled full of substrat pro (no centre core like the aquaballs). Right at the top where the aquaballs have the small box, the biopowers have a small fine sponge. Only the bottom basket has slits. The water is drawn into this basket, through the sponge then up through several baskets of substrat pro, through the fine sponge and out.
Because of the amount of substrat pro the biopowers hold, Eheim are marketing them as an internal filter with external filter technology. The smallest biopower, the 160 (2411) is labelled as being for 80 to 160 litre tanks, but you can turn down the flow rate (and on the aquaball too). It has one basket with a sponge and one basket of substrat pro, The aquaball with the same flowrate as the biopower 160 is the aquaball 130 (2401), though the quoted tank size for that is 60 to 130 litres. There are smaller aquaballs too.
I may sound like an Eheim salesperson, but I do have both filters. I used to have an aquaball 2212 with an additional basket in my 125 litre. Then the filter in my 60 litre started to play up (it was very old) so I got a biopwer 240 for the 125 litre and put the aquaball into the 60 litre. For the 125 litre, I much prefer the biopower. The aquaball is fine in the 60 litre, though I've had to turn the flow down on the aquaball, but then that model was designed for 100 to 200 litre tanks.
