Depends on what you want the fuge for...algea or pods. Pods get into the tank better with a HOT set up as they just over flow into the tank. The HOT modles tend to be pretty costly and I think ugly. Under the tank can be simply a 10 gal tank and it is out of site. Most HOTs come with a nice light, most that have unseen fuge use a screw in type pc from Menards with a cheap shop light type reflector. Under tha tank can be done for about $50, that doesn't even get you close to a HOT. With an under the tank some pods will make it through the pump and come out alive, not nearly as many as you get in a HOT style.
Hope this helps