Round bidet seats
Bidet seats are made in both shapes and they are not interchangeable. An elongated seat on a round bowl overhangs at the front.
Round bidet seats are also less widely made than elongated ones, so the choice is narrower. If you are planning to add a bidet later, that is worth knowing before you buy the toilet.
How to tell which you have
Measure from the centre of the two bolts at the back of the seat, in a straight line to the front edge of the bowl. About 16.5 inches is round; about 18.5 inches is elongated.
Measure the ceramic rather than the seat, since a seat can overhang.
When round is the right choice
Powder rooms, cloakrooms, and any bathroom where a door or vanity would clash with a longer bowl. Building guidance suggests around 21 inches of clear space in front of the bowl — if elongated would eat into that, round is the sensible option.