There are two types of aquarium chillers. They are the drop-in and inline chillers. The drop-in chiller resides next to the tank while feeding coolant through an external probe that sits in the water. The inline chiller processes water being fed to it by the pump.
How do you decide which one is best for you? Well, basically they have the same features so either one of them will work. One deciding factor will be the size of your tank. A larger tank will require a larger chiller. A smaller tank will require a smaller chiller. Chillers range in quality and efficiency. If you need to purchase one you need to get literature from several companies that sell the chillers and compare them.
No matter what the size of your aquarium is, you will need the use of a chiller to keep your tank cool. You can search the Internet for prices on different chillers. Also your local pet store will have some information that they can give you.