I always price out all my internet orders at AHS, Midwest, NB and Rebel, and end up placing my order with the cheapest place that has all the items I want. If everything is within a buck or so, I'll end up going with Austin because I don't have a mill and you can order crushed grain by the ounce.
In general, I find that prices are usually within 5-10 percent of each other once you factor in shipping. I also think that of the four:
Rebel has the best prices on average, but a smaller selection
Midwest tends to have lower prices, but non-flat rate shipping ends up bringing the price back up in line with the others
NB tended to be a little pricier on most orders, but flat rate shipping kept it close. They also seem to have the biggest selection. I find myself buying from them every few months just because they're the only place I can find a particular ingredient. I've also willingly paid a few extra bucks to shop at NB every so often to show my support of BTV.
AHS seems to have the right balance of selection, price and flat rate shipping for me. I'd say I do about 60-70% of my online business through them. Like NB, I find that they have certain ingredients that I haven't found elsewhere.
I tend to buy ingredients for 1 batch at a time, plus one or two random bits of gear, so YMMV if your purchasing habits are different from mine. I've also had excellent customer service from all of the major online retailers, and will not hesitate to shop at any of them.