Maybe both.
and the thing about using an impact as opposed to a breaker bar (in most cases) is that it loosens up the bolt through vibration. this gives you a better chance of a clean removal instead of shearing off the bolt head
if you go right after a rusty old exhaust bolt with a breaker bar and all the oomph you have, chances are that head is gonna break right off. if you dont have an impact gun or heat source, the best thing to do is LIBERALLY spray with pb blaster or equivalent, let it soak for a day, come back with your wrench or breaker bar and start going back and forth gently to try to break the bolt loose
it may also help if you go for a nice long drive to get the exhaust hot first, then try to take it off. i know it would be hot as s*** under there, but you might be able to at least break the bolts loose first with a hot exhaust
i realize youre already having it done, that was just some info for anyone else who may want to attempt this