Expect to pay a little more if you're getting the types of muffler above (pricewise and installation wise). There is slight modification required to lower the exhaust system to aim those mufflers straight out. Not very hard I assume, but an exhaust shop will most likely add more for installation.
EDIT: By the way, I assume the single and double wall has to do with the tip. look at the single wall muffler, looks like a pipe cut off. The double wall appears to have a shell type of pipe for "looks" I guess, I would take the slash cut pipe look. Just me though.