FireWire 800 is significantly faster than USB 2.0, yes. Use it over USB if you can.
You should also have a Thunderbolt / DisplayPort port as well, which I believe is a tad faster than FW800. Google will help there though.
FW's working speed is, in my experience, about double that of USB 2.0. However, if you are using this primarily for backups, and that backup system is Time Machine, after the initial backup then the time it takes to complete the backup is going to be relatively small with both. Consequently, the higher cost of firewire enclosures, and the relative scarcity, plus the fact that you can buy a USB 3.0 enclosure for $5 that is compatible with 2.0, may make a USB enclosure more justifiable.
Pretty hard to find a Thunderbolt 2 or 1 hard drive enclosure or even a SATA to TB cable. I have my old 2.5 inch spinner I would like to use for backups.
It will be hard to find TB enclosures because they entire solution had to be certified in order to be on retail shelves. With that said, I would go with Firewire over USB 2.0 for my backup drive. USB 3.0 is a different story of course. You could work on either, but if you need to boot from your external drive, you can continue to be semi productive on the FW side vs the pokey slow USB 2.
I'll try and find a FW enclosure. Hopefully as cheap as USB.
Will do. It's my old 2.5" 250GB spinner that came with the machine (replaced with SSD). Nothing daunting.What's the size of the disk you're using? Some multi terabyte solutions (3+) get flakey when stuffed into USB enclosures, in some cases.
If you don't mind buying used you can always find something from g-tech on eBay. just search for g-tech g-drive mini fw800.