I shop online mostly, I spend probably 25% in the states. Mostly BRS but recently used Dr fosters for salt as well as being able to get dog and cat things all in one place. My first purchase was a light unit from fish need it in the states for my 20 gallon, cost was the only factor. 24 inch 4 bulb t5ho with bulbs was $180 I believe. Locally I could get a 2 bulb for 150.
Now I shop online for what I need and as long as it's not a huge % difference I don't consider the price to be a determining factor to pick state or canada side. I like J&L for a very fast and free delivery. Recently ordered DI resin and gfo from reef supplies.ca and soon will be getting a wp40 from Nick at reefsupplycanada. Not to mention the money I drop on MJC
But for me online is the only way to go. I spend at most 5% of my reefing dollars locally because of price, selection and quality. Occasionally I see something nice but very seldom.