This is only MY EXPERIENCE:
ARC and SWP are the best, though I have better luck at ARC as I tend to have a more expensive coral taste. Not knocking SWP as I go there from time to time as well. SWP has good things, but from what I have experienced ARC has the more original pieces. I also have better luck with ARC livestock.
Pets plus is pretty decent in lockport, but I have only been there once and they had good things. (Not much experience with them)
Markhiems dosent even compare to that of ARC or SWP. Staff is not very friendly and dosent seem to know as much as the staff at ARC or SWP.
There is a store called the fish place in tonawanda, stictly fish though. Good looking fish but they usually carry the same fish in stock it seems. (Not much variety)
Hands down ARC in my opinion is the best with SWP in second for reef tanks..Any other store dosent compare in the buffalo area.
Sorry for the long post lol..just sharing my experiences, and again thats only MINE, everyone has a different opinion.
As far as online purchases, I say support your local fish store. (Which ever one you choose)