General

Some info on cannons:
Great mobbers, godawful 1vs1 til 4th job.
For mobility we have Monkey Push (basically flashjump) 1st job, Blast Back 1st job (Propels you backwards mid-jump), and Cannon Jump 3rd job (vertical jump).
If you're using push to get across a map, you can do cannon jump in midair right after to go farther. Useful for crossing huge maps.

Out of the explorer pirates, I've only played cannoneer. I'm not going to try to compare them to anything without having tried the others, but maybe this info will be useful.

I doubt there's any significant difference since the fastest way from 1-120 involves (in general) questlines to 70 then RJPQ

bucc as spiral assault is greating for mobbing enemies, then when all the enemies are in a large group just use static thumper

Cannon is your best bet. Sair is the slowest

Cannon Master

I'd say Cannoneer since Buccaneer has to be supercharged in order to use the long range double blast at 4th job, while Cannoneer doesn't need to recharge but still hits that length and actually a bit longer. Cannoneer also attacks a little faster than Buccaneers when it comes to mobbing, but Buccaneers are probably stronger when supercharged.

Edit: I didn't read that you just wanted to get to 4th job then switch to Buccaneer, but I'd still say Cannoneer considering Buccaneers have fewer mobbing skills especially at 2nd & 3rd job.

bucc should be fastest =) or cannon idk @_@