Twin turbos in my mind is better than one single. It allows you to get the same power as a larger turbo but minimize the turbo lag. Two small turbos spool up a lot faster than one, also you can feed one to each bank of cylinders. I've got the new ecoboost twin turbo v6 in my truck and the torque that is coming out of a 3.5l V6 is truly impressive, 420ft-lbs of torque. @ 2500rpm I don't think you would see even close to comparable number with a single turbo setup.