Which solver to use by CST STudio

Hey ,
i need some help. im simulating two antennas in a cavity box, i first do it with the transcient solver and after with the frequency becuz i wanted to see if the results will be the same .. but they are some difference at some frequencies .. i will like to ask which solver should i use ? what is the right for my application ? which results should i trust?

Generally transient solver is used and it is best when design dimension are more then wavelength of operation (its able to solve most of design ) its fast in solving.
Its as per my knowledge