Fabric-store has the consistently best prices. They are able to do this, however, by stocking a cheaper quality of fabric. Their linen uses rough threads in loose weaves (look at the low thread counts and the large number of 'slubs' [little rough lumps]). Tunics I've made from it have looked old and worn after only a half dozen washes (this could be a good thing if you want your garb to quickly look lived in; it's not so good if you want it to last a long time). It also tends to fray more quickly than the higher quality linen I've purchased elsewhere. All which is to say, there are reasons why they're always able to keep their prices low. It's still the most reliable source for the widest selection of colors, and their prices are always reasonable. But it's the low end of quality.
Jobbers like fabric.com and fabricmartfabrics.com stock odd lots that they buy up at a big discount from manufacturers. That's why most of their stock is so inconsistent - they buy whatever's available. That means, however, that they're often able to stock a much higher quality product at a price comparable to fabric-store.com, because they are getting their fabrics at a big discount (while fabric-store.com is buying new from the manufacturer, paying full price for lower quality fabric).
So it's a toss up. You can always count on fabric-store.com to have what you want, in lower (but acceptable) quality. But you'll get finer, longer lasting fabric if you can hold out for a sale from one of the jobbers and can grab the good stuff (the good stuff is usually the European linen, made in Italy or Ireland).