Twin turbo would definitely offer more power output than a S/C but only if you really beefed up the bottom end. But at this point we're talking a shitload of power that I doubt you're looking for. Honestly a twin turbo is for huge boost levels and putting that on the street is a waste of a turbo or an accident waiting to happen. I single turbo would be perfect and spool up faster than a twin turbo kit. Also have more room under the hood when its all set up.
A S/C on the other hand would have a ton more low end torque and be a much simpler setup.
The guys that installed all my parts on my car are running a 600 something cubic inch big block with two 96mm turbos I believe... they are hitting 3000+hp right now.
Twin turbo if you have the money and a good shop in your area. You can go with two smaller turbos and together they will spool instantly, the torque will be amazing, and your potential for additional power will be through the roof. You can tune your horsepower to a reasonable level for a stock bottom end and have a beast (or build the bottom end and make a track monster). Plus you will have a set up that hardly no one else has, making you a unique little snowflake.
I agree with the others, you can't real make a choice until you have an end goal plan. For a DD a super would be easier and give you plenty of power. It's also cheaper and easier to install. If you're planning on putting down huge numbers, turning it into primarily a track beast, and have the cash I would go twin turbo though. Have you considered a single turbo? From what I've seen and heard Hellion makes a real nice kit with a turbonetics housing i think. They run about $5500 I beleive, and will give you a real nice power output, kind of a compromise between the two options you were considering. Twin turbo setups for S197s are stupid expensive, plus as previously stated you'll really want a built bottom end for it.
Thats why someone like me could just chime in and say... tell me what i need to know about these on3 kits. i would like to know what turbo would be the best to consider with a dd, i am set on the paxton when in a couple of months, but do not know much about turbo's that hellion kit good for a dd?
on3 kit is good price if you don't mind not getting a complete kit all in one package. if you don't mind piecing it together then you'll be fine. depending on your power goals and over all goals will depend on what size turbo will be better for you. with a turbo the size is extremely important. a smaller one will spool very quickly w/ virtually no lag while a larger one will not be as good on the street since it will take longer to spool and will be more prone to turbo lag.
most people that are in the old way of thinking will say turbos aren't good for street use because of the lag, but with technology and proper turbo size you will be very happy