Lighter is always going to be faster, its simple physics.
Now, when you have these discussions on Bob's, you will have the argument of "well I run a 350 class and weigh 400 pounds and win all the time". Not denying that, but your not comparing apples to apples...level of competition, setup, tires, etc all come in to play. Ill take 25 lbs over weight vs. the wrong tires any day of the week.
But in a true apple to apples comparison, lighter will always be faster. Momentum racing isnt really a factor...we race on ovals where you scrub speed when you turn the wheel, or pull up hill out of a corner to climb banking. If we raced downhill in a straight line, yea momentum would be great.
In our experience, 10 LBS of overall weight is close to a tenth of a second on the stop watch.