I'm going to throw my 2 cents in on this one.
The 5.0 is as many have said, an amazing car. Any TRUE car enthusiast who looks at the new 5.0 will have nothing but respect for it.
I'm not a fan of old school muscle, but I undoubtedly can appreciate what it stands for and what it's designed to do well. Performance wise, the 5.0 is on point with the M3, and even if you just ignorantly throw out that it might be the case only in a straight line, you'd be blind to not notice the leaps and bounds Ford has truly made from it's previous stang generations in regards to what the car can do on a track.
Based off your situation and the things you've said, it sounds like the 5.0 might be the way to go. The M3 is an expensive car to keep up and maintain, and modifications to it are NOT going to cheap either. Plus... the big picture here, and I don't know maybe it's just me but if you f*ck up in a M3, you'll cringe. Not just any cringe, you'll cringe so hard it might cause internal bleeding... and seriously... you just might die. The 5.0 isn't like that, I'm not saying you can try and slide a 90 degree turn at 120mph roll 800x and feel like a million bucks afterwards, but I'm saying you can afford to make mistakes in a 5.0 and not have it set you back for the next year.
I'm a big fan of Motortrend. They did an article that compared the 2, great read and I was TRULY impressed with the new 5.0.
That's just a Motortrend clip, I'm sure you can grab the magazine if you wan't the full read.
If an M3 is your dream car, keep saving bro, you'll get there. Is there any rush though? ( You're 22? ) Not really. Could you polish your skills in 5.0 till the M3 becomes a reality? Of course.
In the end though, just do you. As Awesome said, it doesn't matter what anyone else says, just make the choice that's right for you.