These descriptions aren't exact but they're relative to the meters given when you look at the description of each gun.
M24(Bolt Action): Accuracy: 5/5 Damage: 4/5 Rate of Fire:1/5 Portability:3/5 Recoil: 2/5
L96A1(Bolt Action):Accuracy: 4/5 Damage: 5/5 Rate of Fire:1/5 Portability:2/5 Recoil: 3/5
SR25(Semi-Auto):Accuracy: 4.5/5 Damage: 3.5/5 Rate of Fire:2.5/5 Portability:3/5 Recoil: 3/5
MSG90(Semi-Auto):Accuracy: 4.5/5 Damage: 4/5 Rate of Fire:2/5 Portability:3/5 Recoil: 3/5
Personally, I wouldn't choose the M24 or the MSG90. The M24 is accurate, but extremely slow and doesn't do the one shot kill. With it's rate of fire, you'll be dead after you fire the first shot. The MSG90 is almost exactly like the SR25 except it does a little more damage but it's rate of fire is noticeably slower. If you're deciding between the SR25 or MSG90, they're both 2-shot kill guns so a small increase in damage in the MSG90 isn't going to be much help. However, the small increase in Rate of Fire for the SR25 is a lot of help. That allows you to nail the inevitable two shot kills much quicker than the MSG90 thus increasing your killing ability. The L96A1 is my favorite sniper rifle (1shot kill). My accuracy with this gun is more than 80%. The only shots I can't do with this gun are the long range targets who are hidden behind sand bags and only have their head showing. Besides that, I can use this gun in close quarters combat [I have a quick shot] as well as mid range and I can be extremely deadly. On snow valley (I'll use it as an example since it's basically a sniper map), I can average about 75 kills with 10 deaths depending on the length of the game and the game mode.
To help you decide, ask yourself a few questions:
1. Are you accurate?
2. Are you a quick shot (able to shoot within 1 second)?
3. Are you able to dodge bullets during a reload?
If you answered "No" to any of these, I suggest you use the SR25. Otherwise go with the L96A1.
If that doesn't help you, then use whichever feels fun to you. It's a game afterall.