L99 is smoother, makes better power more efficiently, and can get slightly better MPG than the already good 6-hole 4.3L Vortec.
More total valve area per CI (In & Ex), an overall better solution, no split journal crank pins, funky odd-angled distributor firing intervals or clever rotating weight adding steps required to "fix" unavoidable balance issues due to the bank angle being 90° instead of the ideal 60°.
It was also the base 9C1 engine. The 5.7L (LT1) option had to be ordered seperately.