In my opinion (an most information on the boards is an opinion) the BC is the way to go.
First off, I don't think the GF is good value. My recent stay at the GF was okay, but I don't think is was worth the cash. Other posts on the boards state the same opinion. It's a beautfiul place, but certainly not up to a 4 star reputation and a 4 star price.
Second, the BC location is superior. If you're headed to MK and EPCOT only, the BC is a very short walk to EPCOT, and a bus that takes you right to the MK entry. The GF is super accessible to the MK, but to EPCOT is a monorail, a switch and wait, and another monorail. Also Future World in EPCOT is getting very dated (think 1980's or 90's world), so the world showcase is more my style. The BC (through the international gateway) brings you in between the UK and France, but the monorail brings you in at the front by Future World.
There will also be way more for you teen to do at the BC with the pools, and Boardwalk right there (not to mention the Disney Studios close by if he/she is interested).
Finally there are far (I mean far) more dining choices at the BC. You have the Boardwalk opportunities, as well as the many EPCOT restaurants within close walking distance.
Good luck and have fun.