I find Fire Mountain to be good for a lot of basic, inexpensive supplies. I tend to get plated chains or cords from there, cotton-lined boxes, simple jewelry displays, Swarovski crystals, and I'll often take a chance on inexpensive or very straightforward beads that look fun. The prices for a lot of things are fairly good if you can get up to the 50+ items range.
JetBeads allows you to order a smaller amount from FM and still get the 50+ item price, or get a small price break if you are ordering 50+ items already. I have used them once, and found it worked it out -- the only downside is that it does add several more days to the process. If I'm already ordering 50+ items, it doesn't seem worth it for the relatively small savings, but if I'm not, and don't need the items super fast, JetBeads would I think be very worthwhile. They were responsive and I had no problems with my order.
Some things, however, either aren't available on FM or are considerably more expensive. If I want sterling earwires, I get them from Fusion Beads; if I want titanium/niobium, I go to Reactive Metals. I hate FM's craft wire options; I use Parawire or Artistic Wire for enameled copper wire, depending on what color/gauge.
I would not buy stones from FM except for the very inexpensive why-not ones; I don't like not being able to pick what you're going to get, and my impression is that what you get can be pretty inconsistent, and not necessarily nearly as nice as in the photos. I really only buy stones in person, sometimes from my local bead shop, sometimes at the Rings&Things show when they come to town. (A great place to be able to see what you're buying and get a bit of a discount.)
FusionBeads is great if you need a small amount of something right away, because there's no shipping or overhead cost; it's all already rolled into the item prices. So if you find you need a single crystal or bead, it's a great place to go.
If you can get to the 50+ level with Fire Mountain, the other nice thing is that they do have a very flexible return policy. As long as you haven't removed things from packaging, you can return them at any time for a full refund/credit. So it is possible to take a look at what you get, and return things you don't like.