use as much filtration of as many differant types as you can afford. The only limit should be your budget and keeping the current in your tank below warp 6 . Diversity in filtration ensures that you can do maintenance without upsetting microbe poulations and water quality. I use sponges, cans, and hang on backs all at the same time.
Spare filters are also good for setting up other tanks or helping out with filtration in a tanl that has lost its bugs due to medications etc.