I would say it is a toss up whether to go Internal card or USB. It depends on which You can spare.
USB is definitely easier. Just remember to follow the installation instructions for which ever you choose. They often need the Drievr disk installed first then the hardware.
What to get depends On what You plan to use the Wireless for, if it is only to share Internet or if it is for Internet and file transfers.
At this time the latest seems to be 802.11ac Dual Band. To get the benefit however You would also need to use a 802.11ac router.
However for most people Wireless N is most likely good enough.