I think the more fine pored sponges and such you have in your filter, the more potential there is for flow reduction, and the more cleaning/maintenance required. Sponges will eventually get blocked with gunk, so you'll have to disassemble and clean an entire filter's worth of sponges routinely, whereas things like Bio Balls, Eheim Substrat, etc. do not need washed. Depending on the type of foam/sponge they do break down and get yucky over time, so you will be replacing them occasionally on top of that. Just my 2 cents though.