Well, with tanks I think you're likely better off locally, if you've got a good shop and can't have a company like Glass Cages deliver locally. Used on Craigslist can also get you amazing deals, if you live in a city and are willing to wait. Shipping on a tank kills the savings, especially if you need a stand.
For fish...prices are much cheaper online, but the shipping is a problem it unless you buy a bunch of fish at once or want something rare. To stock all at once, you just need a very prepared tank (which can be done).
Things such as food, protein skimmers, accessories, etc. are much cheaper online, even with shipping.
Oh, and for corals and cleanup crew...take a look at GARF.org. I have to plug them, they're an Idaho group working for aquaculture of corals. They usually have great prices and an amazing selection, although you'll have catch someone to personalize a pack. I don't know how well the shipping works outside of Idaho, since I'm only about 6 hours away, but I didn't have any problems ordering from them.
Oh and for corals try the local aquarium club. It's quite possible you'll be able to get frags for virtually nothing.
Prudence is a rich, ugly old maid courted by Incapacity.