They are both elongated bowls, which means they can use any elongated seat.
We use the SS114 and SS204 on those.
The Ultramax II is 28" long
The Aquia II is 27-1/4" long.
It's the size of the tank that saves the room, not the seat.
Performance is similar, the Aquia uses dual flush, the Ultramax is a 1.28 GPF
Aquia is a washdown bowl, the Ultramax is a siphon jet bowl with a circular rinse over a Sanagloss finished bowl.
The Aquia is skirted, the Ultramax has the exposed trapway.