Hey Mark!,
1st let me start by saying,I've never marinade a turkey, but I've brined a few this year.
I'll quote from Smokin' in his Turkey 101 update for 2010," The main difference between brining and marinating is simple. Brine are salty solutions and marinades are acidic solutions. The acid in the marinade helps break down the meat."
Both will add flavor, but my thoughts are that properly brined turkeys will have a deep flavor. I personally would recommend you try Smokin's holiday brine. There is a reason most of us call him the MASTER of brining.
If you can't find one of them low solution enhanced birds, I'd recommend going to Wal-mart and see if you can find a 8% enhanced bird and brine it for 48hrs. It works for me!
So I think we learned that marinating tenderizes the bird when adding flavor, while brining adds a "deep flavor" and moisture,buffer to keep from drying out the breast if you over cook.