Rapid fire....as in a short burst, or as in run 1,000 rounds through it without stopping?
The light weight barrels are what the military used on the Colts for many years. Also, look at a M4 barrel and see how light it is. These things run full auto.
A heavy barrel has some advantages if you are looking for something like a match or target rifle where you have to maintain absolute accuracy while firing many rounds rapidly. Other than that, I wouldn't worry.
Oh, I have both styles and both work fine.