I do substitue brands sometimes, if I can't get ahold of the brand I am looking for, at my LHBS. Often times i just use the LHBS bulk LME. THey do differ a little, and have slightly diffrent ammounts of fermintables. It all depends on how exact you want to be. If you are doing a clone beer, it might make a diffrence in the flavor. I normally don't worry about it, as I often don't do clones, I just go for a style beer, and try to make it my own.
So, to sum it up, they do very in the ammount of fermintables, and will very in flavor just a bit. So the choice to substitue is up to you.
If you are going to a LHBS, you should be able to ask them, a lot of places may have 3-4 diffrent Amber extracts, ask them which one would be closest to the one your recipe calls for.
Primary 1 -- Coffe and Cream Robust Porter
Primary 2 -- Vanilla Cream Porter
Secondary 1 -- Hail-Storm IPA
Secondary 2 -- Empty this week
Drinking -- NW Oatmeal Stout, Caramel Cream, Belgian/Irish N-Fusion, House Pale Ale