If you are looking at a mag trip setting of 120A, that will correspond to a motor FLC of between 11A (1100% trip) and 15A (800% trip) for a normal motor, maybe as low as 7A (1700% trip) if it is an energy efficient motor. If you have a motor with an FLC of 15A, you can’t use the 15A continuous rated breaker because the disconnect device must be rated for a minimum of 115% of the motor FLC. So you would use the one rated for 30A (between those two choices). But if you have a motor with an FLC of 7-11A, the 30A breaker may cost you a little more.
It’s teally all a moot point though. You cannot select the size of an MCP in the field, they can ONLY be used as part of a factory built, tested and listed combination starter assembly. So in reality, those decisions were made for you before you even bought the starter. The only thing that you can do in the field is decide on the settings and in general, the starter mfr will have selected the components that offer the widest range of adjustment possible.