This was a question I had to ask myself when the new Mustangs finally came in. Having driven all of them from a base V6 to a GT Premium Performance Pack, my favorite Mustang is the EcoBoost with the Performance Pack. I think it looks great, and it drives fantastic, and of course the all new independent rear suspension and rear wheel drive is quite a lot of fun. However, it was always going to cost more to own the Mustang, even from a fully loaded ST.
The Mustang is a two-door, basically 2+2 coupe with an unusable backseat (you need to be less than 5 feet tall to comfortably sit there), yet it is GIGANTIC in comparison to the Fiesta ST, and weighs MUCH more. That doesn't make sense to me. The FiST is much more nimble, gets much better gas mileage, actually has usable backseats, costs less to buy, costs less to own, insurance is significantly cheaper with it, and it has a whole hatch! Not to mention that although the Mustang costs more, it lacks the Recaros (I absolutely wanted these), no moonroof (I wanted this too), no heated seats (not supremely important, but I like them), and has no touchscreen entertainment or navigation system (I wanted HD radio and Nav). If those options are added to the Mustang the price becomes massively inflated.
Now if I wanted a rear wheel drive coupe and had no other concerns, I would go straight to the Mustang. But I want the best of all worlds, and the Fiesta ST just can't be beat for that.