Considering you are a former Cobra owner my guess is the V6 sound won't satisfy you regardless of shiny newness, axle ratios, or peppiness. Last July I engaged a similar internal debate...I went with the V8 (a beautiful '14 Ruby Red
Brembo) and couldn't be happier, even as more & more '15 are starting to appear on the roads.
For just $300 you can buy the Ford Racing GT500 muffler that will make the 5.0 sound absolutely glorious. It's music the V6 or Ecoboost could never dream of producing. IMHO, the Cyclone V6 simply never sounds good loud and don't get me started on the Ecoboost which Ford engineers deemed had to be "enhanced" by the radio speakers...heck not even Roush could manage a remotely powerful exhaust note on its new performance muffler for the Ecoboost.
And for another $400 or so you can get the Ford Racing tune with "Lopey Idle" (or other tune with a "ghost cam") and now your 5.0 sounds like a goddamn race car. I recently had an SLS AMG in front of me move aside at a red light so I'd pull up next to him (presumably to race)...once he heard my absolutely vicious "cammed" blub blub blub thundering from my 4" pipes he choose to pull away like a grandma in a Prius (in spite of his 550+hp $100k+ dream car that could have smoked my humble sub-$30k albeit 380 whp daily-driver.) Now this is fun that could NEVER be had in a V6 or Ecoboost!
However if you have matured faster than I have, so exhaust notes and stoplight intimidation no longer matter as much to you, let's not forget that the 5.0 Coyote is a truly world-class engine and the Brembo package is very quietly a Boss 302-lite (check out Brembo50.com) while the V6 is fodder for rental fleets. Regardless what the twenty-somethings now type for car rags about "oxcart axles" and how sublimely wonderful the 2015 is, let's not forgot those shills had been lavishing praise on the Mustang's handling ever since the 2005 S197 was introduced and couldn't say enough about the mighty Boss 302 just a year or so ago. Granted a solid axle is not going to give you the best road comfort or supreme road handling, but trust me (as someone who's raced Miata's) an S197 Brembo is NOT going to disappoint on the road or track...unless it's a quiet, cushy ride you're after (in which case you'd be shopping for the wrong car either way).
Speaking of practical, the most important reason to go used is depreciation...best to let the other guy eat the whopping 25-35% depreciation that occurs in the first couple years. Furthermore V8 Mustangs tend to retain value MUCH better than their V6 brethren. Heck 10 year old GT's are still fetching well over $10k while you could get an '05 V6 for less than half of that. If you get bored of the V6 2015 in two or three years you would lose your shirt trading it in, whereas the 2012 V8 would depreciate MUCH less. So now if the 2017 or 18 Mustang really grabs you, you are much more able to make the leap.
That brings me to my last point (sort of) and another somewhat humble opinion: The 2015 S550 is undoubtedly a fantastic platform with tremendous potential...potential still yet to be tapped. Just wait until Ford Racing has a year or three under their belt racing with the new IRS & revised front end! Wait until go-fast bits from the GT350 start to trickle down to the base models. Wait until Ford realizes they may a tremendous error tying the Mustang to the corporate design language, essentially turning Mustang into a Fusion coupe...
To me the best part about an S197 is that it screams MUSTANG! It's not just that the rear quarter windows look to be taken right off the original or how the headlights remind you of a '67 and the taillights recall a '70... It's also that until 2015, Mustang looked NOTHING like anything in a Ford showroom. While new Mustang has been selling well and Ford claims to be pleased, it is just not the home-run they were hoping for...losing to the VW Golf for Car of the Year had to sting! Up to now Ford has been dismissive of the criticism of their styling choices (wrap around headlights right off of the Fusion, which look like pretty much every other passenger car on the road, rear quarter windows taken right off the new Vette, which were taken from a 2005 Aston Martin DB9, side profile nearly identical to a 2008 Honda Accord Coupe, and so on...) Love it or hate it, we are stuck with the current look for another 2 or 3 years, i.e. after production tooling has been amortized. And then Ford will be ready to restyle the S550. THAT is the next Mustang I look forward to!
P.S. I just pray to the car gods that in 2018 a Mustang still comes with a V8 and not a V6 Ecoboost GT! Another reason to snag that 2012! V8's are on the endangered list my friend so get 'em while you still can!
P.P.S. How's that for a response?