Until you start to really know what you're listening for, yes, they do the same thing.
But in reality they can be really different. Until you get a good amount of experience with the ones built into cubase, I wouldn't worry about external for a while. Keep it simple.
Learn to use the built in one and become familiar with threshold, ratio, attack, release, and gain, then when you think you need something else start researching individual external comps.