There are essentially two ways to go:
Stand along potty chair
Pros: kid-sized so they are not intimidating, allows kids' feet to be flat on floor (important, especially for pooping),
Cons: have to clean them each time, bulky so they add more clutter to the bathroom
Ring inserts
Pros: cheap, can have them in each bathroom, gets kids used to being on the "real" toilet so when they're not at home it's less of a disconnect, come in more decorative themes (adds an incentive)
Cons: doesn't work well for smaller children, probably still need a stepstool
We actually started with the stand-alone chair, which I suppose was good for getting my son interested. However, he was a big boy, so by the time he was serious about training, he was already too big for it (didn't fit well over the bowl, and that was without the little pee gaurd!). He actually only peed in it a couple times, and never pooped. We bought those rings instead, and he took to them immediately. We already had stepstools in each bathroom for handwashing/toothbrushing, so it was no problem to slide that over to the toilet. The rings were easy to clean, inconspicuous, and easy to take on vacations or to others' houses.
So, I'd say maybe buy one of each? There's a certain novelty to having the actual chair in the bathroom in terms of getting kids interested. In terms of which chair to buy, we bought a no-frills one (no sounds, etc.), I think from Safety First of something like that, that had a pull-out bowl.
Good luck!