the single is just thin and basicly a stainless steal tube cut and the double wall is 2 pieces welded together inside. ummm. its hard to explain, there is basicly just another piece of the steal inside the tip to give it a nice flush clean look.
yeah there a couple extra pennys but if you love the look of them IMO its worth it. they look much better in person too. i got mine at the muffler shop for a little less than the website.
check out pep boys selection on their website. they have a ton of exhaust tips! i bought i nice bolt on a long time ago for one of my previous cars. It was just like dealer tips they have on some of the new cars. it had allen wrench bolts that wernt all sticking out and lookin all crazy like.