Hi Susan!
Thank you for bringing your question to the Disney Parks Moms Panel!
It's always hard to decide which resort to stay at for race weekends. I love them all!
I honestly don't think one will be much different than the other with regards to transportation. The monorail will be running on the morning of the half marathon. But, there will also be buses running to and from each resort for runners and spectators. Both options would be very convenient for you, with the buses maybe getting a slight edge because you have to change monorails for the journey to Epcot from the monorail resorts.
If I were you, I would choose the one that you like best. For me, we like to stay in Epcot resorts so we can walk into that park easily (it's our favorite). But, I also know a lot of people who like staying on the monorail best.
Disney's Grand Floridian Resort & Spa is also a great choice for
Princess Half Marathon weekend. You can't help but feel like a princess in such a gorgeous, luxurious location.
I hope this helps with your decision! Please let me know if you have any other questions.