They are both made from the same "electric toothbrush" handle. I have 2 RabbiTatts, and after using my friends EZTatt, I didn't see any difference. Go with which ever one is cheaper and or whichever one you can get the fastest.
Practice on a banana skin first. (Yeah, you can still eat the banana, it doesn't go all the way through!). Read the instructions. When you set it up, I find the most important thing to check, is the length of the needle at the tip. Adjust the tiny collar (tube) that guides the needle. You don't want the needle to shallow or too deep.
And change the batteries as soon as you hear it start to slow down. LOL! After about 200 tattoos! (Fresh batteries keep it running best.)