I voted both but it really depends on the girl.
Someone that is nice and clean and no hair and doesn’t have creepy crawlers down there is fun to go down and tease.
Someone who is trashy that you meet at a bar is straight up railing 
As for knowing, you gotta feel out the girl. Many you can feel tighen up when you hit the right spots and such. Some just lie there and you know you are fighting a losing battle.