One thing I can say is do not buy cars out of pure emotion. Emotion can lead to negative equity and high payments that you justify left and right, but in the end are still a bad decision. I can so totally relate to wanting something different and falling in love with a car that isn't the one you own, but in the end it is better to curb those feelings and only make the move when it actually makes sense without roundabout justification. I highly, highly doubt the EB Mustang is going anywhere for a good while so taking another year or two to ensure you're positive on your trade and that you're making the right decision isn't likely to prevent you from getting the EB Stang.
Not for nothing, but you also said "buy the Mustang I really want which is the GT". If you really want the GT and you get the EB, I can almost guarantee you'll be pissed at yourself after the "yay, new car!!" feels go away. I'm not saying you need to pay off the FiST and keep it, just don't rush the situation and purchase the EB when you really want the GT thinking to yourself "well it's still a Mustang" because in the end you'll likely regret it and very possibly end up in the same situation in 2 years when you're trying to get out of the EB and into a GT.
If you want a GT, get a GT because the EB is not a GT and even if you can mod it to be as fast or handle better, it's still never going to be a GT. I'm corny when it comes to cars, but I feel like they have to speak to you, there has to be a connection. Cars have a soul and an attitude, and no amount of modded and tuned power or handling prowess is going to give an EB Mustang the same soul and attitude as a GT, at least in my opinion.
Disclaimer - I'm not anti-EB. I think it's an awesome car and for the right person it is the right fit. It has a huge mod ceiling and can be made damned fast and into a damned good handler, but it is still not a GT.