[82mm 4g63]

Short answer? The Mustang GT.

When you round numbers off and ignore the Base 2.0T to prove your point you're not being fair. Use the actual numbers and available vehicles/engines and try again.

Mustang V6 Base - $23,105 - 305hp
Mustang V8 - $30,105 - 412hp
$7,000 = 107hp = $65.42/hp

GC 2.0T Base - $23,100 - 274hp (+64hp)
GC 3.8 - $27,600 - 333hp (+27)
$4,500 = 59hp = $76.27/hp

Using the current prices for the base GC, you're getting more power for your dollar with the jump from a Mustang V6 to the V8. When you combine the hp/tq per dollar plus the ease of squeezing more performance out of a turbo4 from the GC you would have to be a moron or turbo hater to buy the V6 over the I4.

[Edit ->]
Not to mention the torque increases between the two.

Mustang V6 - 280 lb-ft
Mustang V8 -377 lb-ft
$7,000 = 97 lb-ft = $72/lb-ft

GC I4t - 269 lb-ft
GC V6 - 291 lb-ft
$4,500 = 22 lb-ft = $204/lb-ft