I had both for some time, my son liked having the choices. But when it comes to fully potty-training, I took away the "baby potty" and it made him feel like he was a much bigger kid getting to sit on the big people potty.
We use both styles. Some days my son wants nothing to do with his potty and will use the big potty with this seat. Other days he seems scared of the big potty and will gladly use his little potty. Plus it's nice to have one of these in the car incase we are out somewhere.