I'm certainly no expert but I would imagine that:
the Duratec will deliver more power, cost more and weigh more. It's been widely used in motor-sport and has a reputation for being robust. Not that the Sigma isn't robust.
I've been lucky enough to have had Sigma and Duratec powered sevens and I've a high regard for both engines, mind you I must stress that I'm no racing driver, neither am I an engineer.
I agree with Paul Jacobs, it would be an easier question to answer if we knew the reasons behind the O/P