i prefer to integrate factory fans and their shrouds whenever possible. i believe they pull WAY more air than the aftermarket ones, and are much more reliable as well.
however, they do [as a rule] pull a bunch of amps at start up, so relays, large wire, and updated alternator/regulator are usually required when using them.
still, i feel the added effort of the install and minuscule weight gain offset the gains in cooling performance.