You can get an upper or lower extender. some kits include both to go to a full 2.5"
They do basically the same thing. Depends on if you where you want to install it. The other difference is some of the kits are made of different metals or even a hard plastic-like material. Some people prefer one or the other, but they're all pretty much the same kits. I've used metal and plastic ones and they both were fine.
Go with a good price, good warranty, etc.
You'll have to get an alignment after you have them installed.