I used to make my own with generic enclosures but I have found you do get what you pay for. I opened a WD 2.5" external HDD recently, it's larger than the one I made myself. Inside was fantastic rubber supports for the drive if it is dropped, and a direct USB interface. One less thing to go wrong which is good.
As to the interface read below.
Firewire 800 maybe faster but is depreciated. Ok not really but you need to actively search for devices which support it and it's getting worse. It's biggest backer (Apple) have stopped shipping new devices with firewire. Yes it is a bit faster, but it's not the fastest and I do think that this will be the next interface to suffer obsolescence.
USB 2.0 Ok it's slower than firewire but it's staying. What may replace it (USB 3.0) is 100% backwards compatible with USB 2.0 so not only is it definitely here to stay now, but it looks like USB is going to become even more of the set standard for plugging anything at all into your computer. If you're worried about speed maybe you should consider looking at other software. If you're copying your entire library everytime you do a backup it'll be painfully slow. But if you use archiving software that just updates the new files without reading every single one then the difference may only be a few GB which gets transferred in the time it takes to make a proper cup of coffee.
eSATA. This is an interesting one. My laptop supports eSATA, my desktop has a USB to eSATA converter and I have an eSATA HDD caddy. On my desktop the eSATA drive is no faster than my USB drive, but on my laptop it sustains 50MB/s which is essentially the limit of the connected drive. If you have a device which supports eSATA I highly suggest you look at an eSATA drive.
NAS is another winner for me. There are a few devices now which come with Ethernet ports right out of the box. Plug it in to the network and not only your computer but any other computer can write to it. This makes it very convenient. Speeds will top 100Mb/s which equates roughly to 10-12MB/s depending on network overhead.