the inline 6 in these machines sounds f-ing fantastic. i really think this is the best sounding inline 6 in the world.
BUT, Mustang V8 sounds better.
I agree, there is something about a inline 6 BMW that sounds amazing.....BUT...is it worth $20k??
BMWs (and FGCs in general) are always going to price higher than domestic (or Asian) "non-luxury" cars on a $$$/hp scale. It really comes down to what you value most.
Personally I like the interior in this car miles better than the Mustang GT or Camaro SS. You can also look at the recent thread on this forum where the poster reviewed his Mustang GT and pointed out all the bad body panel fits and somewhat ugly (IMO) shiny faux-aluminum interior trim bits. The BMW is better in those regards I'd wager.
That said - I'd likely buy a Mustang GT, add a nice exhaust, maybe some wheels, and still pocket the change. I'd live with the issues for the sweet V8 power.