I think its easily demolition. For one thing the games are LONG and you DONT SIT OUT. Those are two reason to play it, and why i dont like playing SnD(unless you have a party and strategically are gonna rape the other team)
Plus, the bomb gets planted and defused like 3 times a game if you get raped, and if its a competitive game, up to about 15+ times. Spawn trapping is super easy to do, and it levels you up fastest IMO
I like Capture the Flag now actually, playing smart offense, getting as many kills as possible and touching the flag as many times as possible, and getting returns when possible, keeps it active and interesting and has been getting me lots of XP and CP
If ur gud with high killstreaks and getting lots of kills than demolition is better 4 u cause everytime u get from 40-60 kills ull end up with 15,000-20,000 XP per match but if ur gud at camping and been stealthy i recomend S n D