Malt Extract Substitutions

I'm a new brewer -- I've brewed 3 batches using extract/specialty grains kits that my LHBS puts together. I'm thinking for my next batch, I want to try putting together ingredients myself according to a recipe, instead of buying a pre-packaged kit.

So my question is, how similar/different are varying brands and types of malt extract?

I'll give a specific example: I'm drooling over the 'Dubbel Your Pleasure Ale' recipe on p191 of Papazian's Joy of Homebrewing (everyone here has that book, right? ). I've also recently discovered Brewmasters Warehouse and their awesome-looking Brew-Builder tool. Since I haven't ordered anything from BMW yet, I thought I'd price out ingredients for the dubbel there.

The recipe calls for Briess Traditional Dark malt extract. BMW doesn't appear to carry that particular extract, but they do carry Cooper's Dark and Munton's Dark, both 27.0 (SRM?) with a negligible price difference. Of course I could look somewhere else to get the Briess brand, but I want to throw BMW some business, and more importantly, I want to learn more about ingredients for future brewing.

So, at BMW, or some hypothetical LHBS, I can't get Briess Trad. Dark, but they have Cooper's and Munton's. How would I know which one would be a more accurate substitution for a particular recipe? Or does the recipe specify Briess because anything else just wouldn't be 'right'? Or is it all the same except for the name on the label? Is there a resource that describes different brands/varieties of malt extracts and their characteristics that would be helpful?

Thanks in advance for schooling a noob.

There really is no way to know the make up of dark and amber extracts, different companies I'm sure, use different ingredients to make that color. Both Coopers and Muntons are well known brands. For your recipe, I would buy either one and brew with it, I doubt there is enough of a difference that you would really pick it up. I am sure that either one will make great beer.