Small potties on the floor allow a child who is self-motivated to access the potty and claim it for his own/access it as his own will. I bought 4 potties in total for my dd, plus toilet ring inserts for after she was potty trained. (I was experimenting.) She loved her potties, and they could be moved around the house to accomodate her in the beginning. For ex...in the early days, we'd keep a potty wherever she was playing-- living room, her bedroom, the kitchen. (We live on 3 floors.) Then we gave her plenty of naked time, and she'd go without even asking anyone. After several months she got really good at using the potty, so then we moved them to the bathrooms. After a year and a half, she got to the point where she transitioned to using the toilet with ease. She'd put the insert in it herself. Then 3-4 months after that, she realized she could hoist herself up on the toilet and balance on the edge of the seat, using her hands for stability. Once she started doing that, we put the inserts away.
The toilets we have include the baby bjorn (which we now keep in the car, in case of emergencies.) A talking/singing potty that has a lid that says up/down/open/close and a handle that makes flushing sounds, and a fake toilet paper roll that sings about how you use it. Since my dd started training at 17 months, it was a great intro to the toilet, plus various words and songs. And I have a Royal Potty that makes a bliiiing sound when your child sits on the toilet or tinkles. (However, that potty is too sensitive to any movement, so I would not suggest it.) At some point you may find you have to turn off the noise on these toilets, or take out the batteries, as they can distract from the real job. But otherwise, they are great for a kid who's afraid of the toilet, or needs to learn the concepts the potty teaches. I have a travel potty that folds flat, but since it's been lost most of the time, I don't really use it. The few times I have used it though, it's been fine. I think it's important that once your child is training, you don't put him in a position where he has to be asked to pee in a diaper because you are out of the house. If he asks to use the potty out of the house, then you should accomodate him, imo.
hope that helps.