If it's just for backup, who cares how fast it is? I went through the same dilema, but realized that unless you're doing heavy duty video work or swapping out hundreds of GB daily, you really don't need anything more than FW800, at the most.
If you're just using Time Machine on a USB2.0 external, yeah - the first backup takes a few hours (let it go overnight). After that you're talking a few minutes of background processes every hour, which you won't even notice or hear. Save your money and buy yourself more RAM and a 2nd monitor or something instead.