The only downside to playing legendary is that fact that the weapon update doesn't affect the single player campaign. Therefore, you'll somewhat be learning on different weapons. It's not as if it matters, seeing as nothing on the single player(shield / health) is as constant as multiplayer anyway. My suggestion for sniping is either the team snipers, OR play a single player on legendary on one of the maps that has the asshole jackal snipers. Those will teach you real fast and in a hurry to keep your head down and when to take the shot. Their heads are also usually smaller targets than the spartans / elites of xbox live, only problem is they don't move the same.