I've ordered from a lot of them, and I have strong feelings on the matter...
Morebeer sucks donkey schlong. Sorry, boys, but until you fix your inventory system, I won't even log onto your site. I can't tell you how many times I've tried to place an order there, and the stuff says it's in stock, and I place the order, and then I get an email days later telling me that the entire order was held up because one of the items wasn't in stock.
Midwest doesn't have flat shipping, which kinda sucks, but I recently was putting together an order that was mostly specialty grains, and I did a side-by-side comp between them a few others (BMW, NB, AHS), and due to their relatively cheap grain prices and excellent selection, Midwest was still cheaper after the $13 shipping, and they had everything I needed. Most other places didn't have all the brewing salts I needed (AHS, BMW), or, in the case of NB, their per-pound grain prices were so high that it negated their flat shipping.
However, back when I was building my recirculating whirlpook IC march pump setup, I did some serious comparisons between all major online retailers, and Northern Brewer had the best selection and the best prices, at least on the stuff I was ordering.
AHS's selection is alright, but they always seem to be missing some odds and ends that I really need, and it kills the order.
I have to say, NB and Midwest are my go-to simply because their selection is usually all-encompassing---when choosing between the two, it usually comes down to the final price on that particular order. I'll create identical shopping carts at each place (and even include other places if the order is big enough, until they don't have a few of the items I'm looking for, then they get knocked off the list)...then I'll just choose whichever is cheaper including shipping. This past time, due to the grain prices, Midwest happened to be cheaper. But that's not always the case...and they don't have quite the selection of equipment that NB does.