We use one that goes on top of the toilet, too. Mostly because I don't like having to clean the potty. We also own a Unce Upon a Potty Potty (
http://www.amazon.com/s/ref=nb_ss_ba?url=search-alias%3Dbaby-products&field-keywords=once+upon+a+potty+potty) and I like it because the guard in the front is high enough that even a boy doesn't splash out when he pees. It was the wrong fit when he was tiny, though. (I wouldn't recommend it for babies under a year.) We used to have one like the Baby Björn, and used it backwards, with the baby facing the back panel, so he wouldn't splash out.
Now that my boy is walking and running, our favorite is to stand on a stool in front of the big toilet. His brothers showed him how to do that trick and he likes that a lot. Once I found him squatting in front of the toilet getting ready to grunt, so I had to quickly put him on the potty.
He's still learning.