DateTime::strftime works on DateTime objects, while POSIX::strftime works on the parameters you supply each time you call it. Apart from that, there should be no difference between them:

use DateTime;
use POSIX qw/strftime/;

print DateTime->
    now->
    set_time_zone ('Europe/Madrid')->
    strftime ("%Y %m %d, %H:%M:%S %Z\n");
print strftime "%Y %m %d, %H:%M:%S %Z\n", localtime;
__END__
2006 10 09, 14:57:36 CEST
2006 10 09, 14:57:36 CEST
[download]