The only reason I wouldn't always use the L96 is because of its below par fire rate and recoil. It isn't a problem for me because I average 20 kills with a sniper per game in snow valley CTF. But, for rookie snipers the L96 is not a recommended choice.
I prefer the MSG-90 because I own with it, but it isnt a one hit kill unless you headshot or hit in the back.
So in conclusion, stick with the MSG-90 until you rack up a good amount of kills per game, then switch guns.