I am currently using a refrigerator for mine, because it was free. I had bought a chest freezer planning to use it, but the free fridge came along so the freezer ended up being a fermentation chamber. A lot of upright freezers have the cooling coils as shelves, which aren't removable, so be aware of that. I like using the fridge because it has a smaller footprint and the top freezer can be used for extra freezer storage for food or hops. My chest freezer is now a lager/fermentation chamber. Another thing about the fridge is the fact that you don't need a temp controller like you do with a freezer. If I were to setup something with intention of using a commercial keg, I might go with a refrigerator to avoid lifting it down into a chest freezer. If it were just for corny kegs, a chest freezer allows easy access to all kegs without having to take others out to get to the one's in the back, like I have to do with mine.
With all that said, price would be my main determining factor. You can get really good deals on Craigslist and might even be able to get one for free. After all, you're just going to start cutting holes in it.