You know, I've always wondered the same thing. I have both--in fact, I have 3 sets of bands (Xmas gifts, RT gifts, yadda yadda yadda) & one set of tubing.
I will say this: I def. use the bands more than I use the tubing. In fact, I almost never use the tubing. BUT I think there are a lot of strength building exercises a home exerciser can do at home w/tubing that I do at the gym w/machines. In fact, I believe STS has some modifications w/tubing.
Aw crap. Just get both! They're relatively cheap compared to most exercise equipment.