Hi Jill!
Your first trip to Disney! How exciting! Magical moments await you and yours on your upcoming trip!
To answer your question:
It depends on the type package you got, and your plans when you get there. Making strictly an apples-to-apples comparison, you would need to invest at least $1200 on your room to get a $300 return, the cost of the potential gift card you are looking at.
My recommendation is this: If you chose a dining plan with your hotel reservation, I'd stick with the 25% off deal. Not only will you be saving on the room, each dining plan comes with a certain amount of savings as well, that usually equals to a pretty good bargain.
The $300 Disney Gift Card deal requires that you make an advance reservation that includes the Magic Your Way room and ticket package. If you went with that deal, that would leave you with the card to use for things such as gifts, souvenirs, and dining.
So there are a number of options on the table:
If the cost of the room is substantially less than $1200, take the $300 gift card deal.
If you didn't get a dining plan option, take the $300 card deal and use it up for dining and other items.
If your room is around the $1200 ballpark, and you don't plan to spend much on souvenirs and dining, take the 25% off.
Hope this helps with your plans! HaVe a magical time!