The worth of a car is not measured by engine size, horse power or even speed. It's all about build quality, materials use, overall driver enjoyment, service, residual values, safety, handling, and much more.
What you have with the Ford Mustang GT is a cheaply made Ford. Just look at the amount of cheap plastics used and other low quality parts in the poorly designed interior. The mechanicals are awful too. The suspension and rear axle are horribly dated, any kind of electronic stability and traction control is old fashioned.
In terms of brute force, the Mustang is the clear winner. In a straight line it will, as you put it, own the BMW (although really we are talking less than half a second difference 0-60). But throw in some real life driving, with turns, and you'd immediately appreciate the BMWs poise and handling. versus the clumsy Mustang with its single rear axle.
When you sit in the BMW, you'll find a classically designed vehicle that has won top ten ranking by Car and Driver for something like 19 years in a row. You'll see expensive wood, and quality materials. Everything you touch has a feel of quality to it.
You can expect good reliability from the BMW, it will hold its value well, and for the first 50,000 miles you can expect free servicing, even oil changes, brakes and wiper blades replaced for free.
Finally, if you are a grown up, you will realize that in this day and age, racing around in a muscle car so fast you can out accelerate a BMW 335i, is just going to end with bad things. Tickets, loss of license, jail, fines, you name it.
For most normal people, they are going to use their cars for a daily commute to work, and perhaps a trip at the weekends.
The BMW will ultimately be a wonderfully fast and rewarding driving experience. The Mustang on the other hand will always be a young boys racer. The harsh antiquated suspension, noise, and over the top styling, combined with that nasty interior, will grow old very quickly. And when you sell it, don't expect to do well there. Fords are not great at holding their value. When you try to sell your Mustang, there will be a dozen others just like it, helping to keep your value down.
Finally there is insurance. The Mustangs reputation as being a car for immature males with a racing fetish will ensure they stick it to you on the insurance premiums. The in your face styling will also make you a cop magnet.
Sorry, but there is no real comparison between these two vehicles, I'd take the BMW any day over the Mustang, and gladly pay the extra for that kind of quality.