I woud say HQ is better for xp but KC is better for getting xp & messing around with your mates.

i only play KC.  took me 5.5 days to reach 10/80.  tag whore, run specialist, pay attention to your challenges.  i even switch out my tatical to max out xp.  kd is 1.3.  i didn't care if i died getting tags cause i knew i would get 300xp the first kill with hardline w/ specailist.  out all run and gun.  some times i would ****, some times i was negative.  hell a fun though.  

KC for sure.

In HQ you still need a good team to actually capture/hold the objectives.

In KC you also get 250 for picking up your own tags after dying. Run tac insert and pick up your own tags when you die and maybe get the payback/confirmed kill at the same time. 350 xp multiple times per match.

Not to promote dying but the more you die the more xp you will earn haha.

Search & Destroy!! No doubts...

Kill confirmed is good. S&D when you are beasting. HQ was the game mode in black ops. But its garbage for ranking up in this game.