EASA licence (UK CAA) can, in theory, be used throughout Europe.
CASA licence can be used in Australia.
EASA licence concentrates on systems and theory.
CASA licence concentrates on anally retentive rules & regulations applicable, in a lot of instances, only to Australia.
Depends where you live, what passports you hold, where you want/can work etc and what licence you're talking about but if it's a professional one then you're better off going for an EASA licence. It'll take about the same amount of time and the nav/flight planning is similar but the Oz air law is just dire.