I've bought stuff from saltwaterfish.com but don't recommend it. They don't have a customer service number and if anything goes wrong you run a high risk of not being able to talk to someone to fix it.
I've heard people mention problems with live aquaria as well. I think that the story that I heard on here (you can do a search) was that someone received a fish that was DOA and live aquaria replaced the fish. The second fish arrived DOA as well and they refused to return his phone calls or deal with the situation.
All of the online stores seem to be equivalent when things go well. I think that you'll find that there's a difference between the various online stores when things go wrong. If I were you I would check to make sure that:
(1) They have a number that you can call
(2) They have a good rep at the better business bureau and
(3) Nobody on here or reefcentral has had too many problems with them
I send an occasional email to saltwaterfish.com telling them how much of my businesss they lost because they refused to fix a $10 mistake. I also went through the BBB in Florida and they did not respond to the complaint.
Also remember that with all the keep alive guarantees you need to mail the fish back to them (at your cost) packed in salt. You may also need to mail them water samples. They suggest that you use registered mail to make sure that it doesn't "get lost". They'll send you a replacement but typically you have to pay for the S&H unless you order more stuff from them. Some places need you to call within 1-2 hours after delivery to report problems. Its all made as complicated as possible to discourage anyone from making use of the keep alive guarantees.
I'm really tempted to buy live goods online but I've been resisting the tempation. I do have an awesome LFS
about 5 minutes from my house though and another one about 1/2 hour away.