haha, With today's technology, it's hard to argue that internal drives are "faster" than external drives. Whomever told you this is probably referred to the transfer speeds that USB or Firewire bring to the tablet, versus transferring everything over your internal SATA drives. If a lightning bolt hits your house, everything is toast.
I have 4 or 5 external drives I use for all different purposes, but I would definitely recommend a 7200 rpm drive over a 5400. Internal drives are generally LESS expensive than external, because external drives you also have to buy the enclosure. Unless of course you buy a universal enclosure for swapping out hard drives on a regular basis.
Iomega is a solid brand, owned by a major company, EMC, that configures enterprise storage drives. They will have a higher price tag because of this branding. THey also have a lot of EMC custom built recover software included if you end up going with a NAS.
Seagate has terrible hard drives that are prone to early life failures. I would stay away from them.
Toshiba and Western Digital are also solid brands for external drives.