Seen from an optimizing viewpoint it doesn't matter.
Seen from a code style viewpoint I prefer < . Reason:
for ( int i = 0; i < array.size(); i++ )
is so much more readable than
for ( int i = 0; i <= array.size() -1; i++ )
also < gives you the number of iterations straight away.
Another vote for < is that you might prevent a lot of accidental off-by-one mistakes.