Thanks all. I drove the ecoboost tonight. To be honest, it's kinda 6 of one, half dozen of the other.
The eco is a premium that's completely loaded, ticking all the boxes. Driving it was nice, it was quick, and it was a "pleasant" car to drive. I think I could be very happy with it for quite some time. It's 5 years newer, but has a bit over double the miles (at 30k). Also, at Carmax (where it's located), they give extended warranties that covers all major failure points.
The GT is a base model (except the CS package). It's not as optioned, but either one is a far step up from my current DD (a base model Outback). It's 5 years older, has no warranty available, but also only has 13k miles.
At this point, it's whether or not I can live without a V8 in quite possibly the only Mustang I'm going to ever buy, and whether or not I'm brave enough to buy a 7 year old car without warranty.