It depends...
Documentation says,
"If you use array_push() to add one element to the array it's better to use $array[] = because in that way there is no overhead of calling a function."
Source: http://us2.php.net/array_push
So it boils down to how much data you want to cram into that array at any particular moment.
Additionally, there's a fall-back. If the array-referenced doesn't exist when you call it using array_push, you'll bump an error. If you use $array[], the array will be created for you.
