Quite a few differences between Leisurepro/Scuba.com and Scubatoys... I've done business with all of them, so here's my opinion:
Leisurepro has a huge selection, but 75% of it isn't covered by a factory warranty. I've also had negative experiences with their sales staff, and often the item isn't exactly as advertised.
Scuba.com is the same as Diver's Discount. They are, at least, an authorized dealer for their products, so warranty shouldn't be a concern. They were the first online company I gave business to, and I didn't have any complaints. I haven't used them for years though...
Scubatoys is an authorized dealer for the products they sell online. They also have great prices, and are extremely knowledgable about the products they sell. They are more then happy to discuss all of your equipment choices with you, and help you decide on what is right for you. Do a search for them... you won't find someone complaining about Scubatoys.. and if they did, I guarentee you that they didn't speak with Scubatoys about resolving their problem.
If you're hard pressed to buy Scubapro/Apeks/Aqualung gear, and aren't worried about having a factory warranty (although it's -very- valuable), you could use leisurepro.
However, if you're after a factory warranty.. give Scubatoys a shot. Great company, no complaints.
-B.