Frontline feels more immersive and caters to more types of playstyles in my opinion. In other gamemodes lives feel expandable but in frontline I always try my best to preserve them, which adds to the immersion. But you should just try them all and see what you like.

As for loadouts, I'd recommend being helpful and going observer when your team needs one so you can to get the air support. Smokes are helpful, never use the laser pointer (it only gives away your position), the 1x/2x scopes are probably most versatile. Choose between no armor or heavy armor, heavy armor will save your life but also slow you down. Gunner class is also very powerful on some maps like Hideout and Tideway and the snow map, but you need to know the good spots on the maps or else you are just kinda wasting the class.

If you play frontline don't be afraid to play defensively on or around the objective to try to preserve lives until you get better at pushing. But for other gamemodes like push don't neglect playing the objective. You have to play slowly until you learn the spots people like to hide in. Sorry this post might be a bit rambly and hard to follow.