If you plan to spend your evenings walking "around the world" or plan to have most dinners at World Showcase, the I would definitely choose BWI. It is so nice to take that stroll or the boat trip -- and so much less crowded than taking the monorail over to the main entrance. This pretty much works for MGM, too.
If you plan to spend lots of time (MK/dining on the monorail) in the evenings around Bay Lake, I would pick the Poly. That way, you'll be closer to your resort at the end of the day, and it would be more convenient if you want to swim or rest in the afternoon.
Alternately, ask her what theme she thinks is more appealing. You really cant go wrong with either choice, I think!