Frankly, nearly anyone who buys a cobra is into "nostalgia".
Tom's right. If you're into performance, the best option (among Ford offerings) is a small block.
That said, for all the reasons that Jason lists, a Chevy LS engine probably makes the most sense; cheaper/plentiful, cheaper aftermarket parts, lighter, and more powerful (and blasphemous in a Cobra, maybe more so than a 385
).