I was recently torn from my 2005 STi when some asshole hit me from behind and totaled it 3.5 weeks ago. I'd had my eye on a Jeep for some time and now I'm getting one.
I don't regret getting the Jeep, as the decision was made for me, but I do regret that I wasn't able to make that decision myself. I had already reserved myself to the fact I was going to have to get rid of the STi if I'd wanted a Jeep, but I just wasn't ready to do so...
Just before the wreck my wife and I were talking about the STi and how much I really liked it and I just mentioned, "as much as I want a Jeep, I just can't part with this car for one right now." 10 minutes later, I didn't have to worry about whether I'd have buyer's remorse.
Anyway, that being said, I severely miss my STi but I'm hoping to fill that void with the Jeep in terms of the fun-factor. No, it won't do 0-60 in under 5 seconds, nor will it hang a corner and zip down back roads, but it will lead me a lot of places I've never been.
That being said, if you're a total Muscle Car fan and have the constant need for speed, you're probably going to regret getting your Mustang for a Jeep. I, on the other hand, also own a 1994 Camaro with quite a bit of work done to it, so I still have that Muscle Car "thrill" available to me.
As much as I'm sure I will love my Jeep, there's no way in Hell I could live without something fast, too.
Despite my misfortunes, to say I'm excited about getting a new Jeep is an understatement. It'll be a lot of top-down fun and trips to basically wherever I wanna go. I'll also have the advantages of having more room for myself, passengers, cargo, and it'll overall just fit my needs better than what the STi did. After I get the Jeep, I may just find I enjoy "Jeeping" more than I ever enjoyed the STi...but we'll have to see.
Nobody here can really tell you whether or not you're going to like a Jeep more than your Mustang. You're going to have to make those decisions for yourself.