Just bear in mind that should you do a mortar course (for example), the powers that be will not be too keen to watch you swan off doing course after course and you will end up being a full time Mortarman. Certainly in the RM, once you have specialised its very difficult (almost impossible) to move from that specialisation.
I would suggest that you wait until you finish your basic training then get a feel for the Unit you are with and how they operate then decide.
Once you have been in mortars all the rest are crap.
But to be serious it depend on your career, I did signals at first and into Coy HQ then back to a rifle platoon and did SF then training recruits and much later MFC and fleet manager. Courses will depend on what the coy/Bn needs you to do not always what you want. But I know mates who did mortars from start to finish and others who did rifle coy to recce to rifle coy.