I don't mind using USB2 over firewire if the cost difference is substantial.
Best to start from basics...
It is either USB2 or Firewire - both are used to connect two devices together so that information can be passed backwards and forewards. Firewire permits either end of the connection to be a master, whereas USB2 has a master and a slave which cannot be interchanged. Without going to into technical details, for most users the differences between the two are negligable.
Next item on the list - how are hard drives connected to the mainboard in the computer? Historically this has been using a parallel connection (PATA), however, more recently the SATA standard has become more common. The reason being that there are fewer wires making cable management simpler. An extension of SATA is eSATA which enables hard drives to be located outside of the computer case without needing to use USB2 or Firewire connections (the advantage here is that the fewer protocol conversions the less chips required and also the quicker data can be transfered).
Connectivity to the actual hard disk is using either PATA or SATA (and SCSI - but that is less relevant these days). The disk itself contains the rotating platter(s) and heads to read the data, plus associated logic so that the disk drive nows where to write and read data. Better drives typically have higher rotational speed plus onbaord cache memory (8Mbytes is good) to make transfer of data more efficient and quicker - however, this does come at the expense of cost, heat and power consumption.
A very simple external hard drive is just a hard disk in an enclosure with a power supply and some chips to convert SATA to usB2/Firewire. This is just a convenient method of locating the hard drive external to the computer and providing simple connectivity.
Moving beyond the simple hard drive then we move into the category of storage systems. This is a box with a number of hard disks with some computer logic placed at the front end to improve the reliabilty, speed and/or usability of the storage. Without going into the details, a storage system uses its internal logic to increase the reliability of the storage by spreading the data across a number of disks in a redundant fashion. It can also add additional featuers such as the ability to create network drives with user or folder based disk quotas so that one individual cannot use all the space on the disk.
What I ultimately want to get to is that the hard disk market is a commodity market - you will end up paying for what you get, or getting what you pay for. There is little practical difference between USB2/Fireware for connectivity or between individual disk manufacturers. A disk is an item with a limited life and will fail at some point (whether today, tommorow, or next week). For each manufacturer you will find someone with a horror story, or others claiming that a certain manufacturer has the most reliable equipment. The only way to be really sure is to use a storage system with redundant array of disks and daily backups of the storage system (one of the companies I worked for experienced failure of the raid controller which wiped out all data on the redundant disks!! do not assume that raid will totally protect your data).
I hope that helps. The main rule with storage is that paranoia rules - at some point in time something will fail, and if you are really paranoid then the backups are also likely to be unreadable as well. The skill is working out where to balance the cost of backups of backups against the value of the data that you are protecting.