Generally the Ashes is more significant to Australia, but you have to consider that the WC is inclusive of every team in the world, while the Ashes is just England. Beating everyone in the world at football is always going to be more significant than beating England at cricket.
A better question might be: cricket world cup or football world cup, and in that case, I'd probably have to lean towards cricket, but only just.
I know a place where a royal flush
Can never beat a pair