I had a bad experience with Sewing Machine Outlet. Their website took an order for merchandise they did not have, because it was discontinued, then charged my card for more than the checkout amount. A few days AFTER they charged me an amount I did NOT authorize, a sleazy guy called me to tell me that they didn't have the item I ordered. He pressured me to accept a substitution which would, of course, cost even more. He really didn't want to take no for an answer. When I asked why I was charged for an item that was no longer available (since I've never been charged for an online order until it actually ships), and why the charged was more than the checkout amount, he said it was for shipping, even though the item I ordered was supposed to have free shipping. Anyway, I made it crystal clear I wanted no substitution and a prompt refund. I called my card company right away, but it took a few days to get a refund posted to my card, for an item THAT THEY NEVER HAD IN THE FIRST PLACE.
So I'd avoid them like the plague.
I bought a refurbed Pfaff from Brubaker's in PA and some accessories from SewVacDirect and had prompt, hassle-free transactions.
There are reviews of the 6300, 6500, and 6600 on PatternReview.com. Janome now owns the Elna and Necchi brand names and there's an Elna version of the 6600 (and maybe the others too) and Necchi versions coming soon. Cosmetic differences only as far as I can tell online.
Best of luck and enjoy your new machine, whichever you choose.