For the best overall performance, properly jetted SU's will get you good torque and good high end power. They are easy to drive on the street too.

Tripple side drafts are best for high end power but their wide open design and manual progression make them a little harder to get used to. You can't just open up the throttle at any time and expect them to take off.

I supposed it also depends on your cam. If you have a mild cam, then get the dual carbs. If you've got a long duration cam for high end power only, then go with bigger tripple carbs.