1.) Buy Molt Gear's Bad Grammer DVD. Watch it and decide on the call you prefer. IMO you cannot go wrong with these three calls: TG Real Thang, TG G Overhauler, or MG EX3. They all can do everything that scott does on his DVD.
2.) Watch the DVD some more with the call you bought. You may sound like crap with a short barreled call at first, at least I did. It takes time to learn how to manipulate the air to get the call to sound right and like many different geese.
3.) This is what I did. I would practice honks one day while driving at work, then clucks, then moans, then murmurs, then trains, then hiccups, then spit notes and quick spits. Now I just practice doing them all in a string like calling real geese. However I find myself doing the train most often because I think it sounds crazy awesome.
Overall, I believe the key is to find a call that can make every note even though you may not be able to on it. Look for a call with a straight bore, short barrel, and broke in guts. Thus you manipulate the call and everything you put into the call is going to come out of it as apposed to the call manipulating your air flow and limiting you.