The twin turbos eliminates 'turbo lag' the smaller one spools up quicker as you hit the gas pedal. I was part of a focus group that got to test drive an '08 6.4 PSD with and then without a 10,000 lb trailer attached.
Pros of the '08
No turbo lag, power is smooth and instantly available.
Quiet, you can actually talk to someone next to the motor, while it's running.
Softer suspension, rides like a cadillac. Easier ride but loses some road feel.
Cons of the '08
Even though I love Ford trucks and own several I would wait until '10 model year so all the bugs can be worked out.