What is the difference between different rated RCCBs except for the price?
In general: An RCD should be able to handle more current then the nominal Amperes of the RCB. The more the better since losses are lower and the chance to destroy/damage the RCD via an overload is lower.

Some versions are most probably more expensive (even having lower Ampere ratings) because of much lower production numbers. In many countries a RCD protects more then 1 circuit, so a 40A or 63A version is standard.



When a second (or more) parallel circuit are added alongside the first 16A circuit (if the upstream installation is rated accordingly), the RCD would protect both circuits. Of course, neutral and ground/Protection Earth must never be connected downstream of a RCD. An overload in both circuits at the same time is very unlikely.

A 25A30ma Residual Current Device can handle 25A in normal operation.
This Ampere value should be always greater then the breaker's Ampere value. 

Here is the reason:
A Residual Circuit Breaker RCB of 16A is NOT specified to interrupt immediately when more then 16A is flowing, e.g. 18A. 
But it interrupts after certain seconds/minutes taken into account that short time higher currents must be tolerated. F.e. a 16A RCB could interrupt a 20A current only after 40 seconds. But of course it must interrupt every shortage with high currents as soon as possible (hundreds or thousands of Ampere in the milisecond range).

Note: if there was a shortage in a circuit, the RCD should be exchanged since it could have been damaged by the high short current.