When considering buying a new fish tank, consider the space you have available and where the aqauriam will be located. Remember, a tank gets heavy. Water itself is a lot of weight. Add in rocks, decorations, filters, and the fish, and you're talking about something you're not going to move very easily.
Be sure when positioning the tank that it is a place where it is visible. Why spend money on an aquarium if you are barely ever going to see the thing? It is also good to consider your tank maintanence when choosing a tank and location. Will it be easy to access the filter when you want to clean it and change the filter cartridge? Is it in a location where you can do water changes with ease? The harder it is to do regular cleaning, the less motivated you'll be to maintain the aquarium.
Also keep in mind that you're dealing with water...and spills, splashes, and accidents will happen. Don't keep the tank near any expensive furniture or electronics.
Just some tips to help if you're acquiring a new tank.