Concentrate on one Job first or diversify?

[MistakeNot]

I'd also say, play how you like, but I did read somewhere you get a better exp bonus the higher your main class is compared to the alt class' you level.

I think that was true once, but not any more. Nowadays you get the same armory bonus regardless of how much higher level your main class is. (+100% exp if the lower-levered class is below level 50, then +50%)