Purchasing an aquarium for piranha?

I have a 45 gallon corner tank for my 2 piranhas (a little over 1 year old), and just started looking at one day getting a much bigger tank. The tank I have now was given to me. I want to put the piranhas in a 150 gallon tank one day, and was just wondering what the difference is between acrylic tanks and glass ones? Also does anyone know if you can stick a tank that big anywhere, b/c 150 gallons weighs a lot. Also while i was checking out tanks online, I see stuff about "overflow" and sump tanks...anyone know what that is and if you really need that?
Answers:
Acrylic is much lighter than glass and allows for clearer viewing, but is susceptible to scratching. However it is much stronger than a glass tank. Acrylic typically cost more as well.
The overflow and sumps are more for saltwater aquaria. If going with a freshwater tank there is not much of a need for them. Basically it is for increased filtration, algea control, and hiding the heaters, filters and what not. Makes the tank look more natural, as their is a lot of equipment used when running a reef tank.
You may not want to put a tank that big on a second floor without first making sure the area is designed to hold that weight. When filled that tank will weigh roughly 1200 pounds. Setting it along a load bearing wall should be ok, but better safe than sorry.

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As far as looks and visibility there is not much difference between acrylic and glass tanks. Acrylic tanks will not crack, break or shatter like glass tanks may, but they are prone to getting scratched. Larger plecos may mark the surfaces of acrylic tanks with their teeth as they eat algae, so are better kept in glass tanks. Acrylic tanks are more sturdy than glass in larger sizes and the bottom does not need to be as completely supported as a glass tank does. You can put an acrylic tank in a place where it will only be partially supported, whereas a glass tank of that size must have the entire bottom completely supported or you are risking cracking.
Overflows and sumps have to do with the kind of filtration you will be using. You will need to make sure you have a good filter for a tank that size and you will probably want some way to automate your water changes. Sump pumps pull water up from the bottom of the tank and can be used to either circulate it or remove it for a water change, depending on what you want to do with it. They can pull dirtied water into a separate "sump tank" which can be easily emptied to help keep your tank clean, or they can be filled with some sort of filter media. Once you get into larger sizes of tanks, the types of filtering systems available become much vaster. That doesn't mean they are necessarily any better than the kinds available for smaller tanks, but you do have more options to consider.