I have bought one that looks like a toilet and it has a removable seat that can be placed on an adult toilet.
i bought one little mini potty that looks like a fun potty and kept it downstairs and then bought a kids foldable toilet seat for upstairs, it saved us having to upstairs everytime we were downstairs and lil man needed a wee
It'll ultimately depend on the kids. I liked the OXO convertible because you can set it over the full toilet or fold down the legs to take on the go with plastic bags, but almost a year later and my daughter will now only sit on her other mini potty chair.