Hi, It depends on some things like, for example, the size of the tank and the type of fish you will put in it. In general, though, for a regular 15-20 gallon tank, it's good to have both kinds of filter since you can get cloudy water and lots of problems if you have only a regular side filter. Decide which fish you want to keep before you buy and set up your aquarium.
So, what size is te tank you're looking at and which kind of fish do you want to keep and which books have you read fish and fish care and setting up an aquarium?