ok deacon91 serious answer for you that will annoy some but is my oppinion...
B Mechs do indeed get a larger scope of postings, u will get B Mechs at every unit even tanks, arty, engr and armd inf units in the HQ's working on B Veh's and at FR units and sigs etc working on CVR(T) and 432's (Bulldog now isnt it?) You will also have much more chance as a B Mech doing cyprus and belize and those other postings that A Mechs dont generally get.
As an A Mech you will probably be destined as has been said already with 2nd Line (as u would as a B Mech) and 1st Line Armd units. Think UK, Germany, Canada if your lucky and all the op tours Afghan and Iraq that go with those units.
Training is very similar but i do believe as an A Mech you delve heavier into engineering that will stand u in better stead long term. Most A Mech's will happliy turn thier hand to B Veh's and trailors if they need to. Show a B Mech a hydraulic system on a CRARRV an he will probably not have an igloo through no fault of his own.
Spaz you can do the pack in pack out arguement all you want. As a B Mech they stop you doing in depth repairs just as they do A Mechs.
When did you last do a syncro change on a LR gearbox? When did you last change a crownwheel in an axle or set up the back lash? When did you last change a valve on a TUM cos it had dropped one? U do the same major assy changes as an A Mech does, Gearboxes, Axles, Engines etc etc.
Could you fault find a gearbox problem on a Wr or a CR2? Or on a CRARRV winch that will not free spool? No, i doubt it. And before you come back yeah i am quite positive i could diagnose a fault on a DROPS gearbox or a Foden Winch, and they are B Veh. A Mechs do go on and do a slightly more complicated level of VM'ing. As much as you will no doubt come back and argue to differ.
Changing brake pads, axles, engines is stuff that any decent tradesman could do and im sure most B Mechs could change a pack on a Wr or a CR2 if they had to. Most B Mechs when i was about however would not have anything to do with armour. And as a VM in an armd unit, we did do electrics also due to a lack of electricians. Though i admit with assistance from the ECE's for confirmation.
The last part again no doubt some will come back and argue about, but i can assure you that anyone that knows me will back up the fact that i would flip if a VM went running to an ECE to try and solve an electrical problem on a tank if it was in the hull. It was our job and we would diagnose and if it was maybe not 100% we would go to the ECE with the facts and figures before we condemned a part to save time wasting and demanding of a wrong part. It was good for the learning process.