I always crochet with the "yarn over" (the "V") because that's how my mom taught me, but I love both! Now that I've seen the photos you've posted, since I want to make a little Batman keychain, I'm thinking about using the "yarn under" technique!
I always try to yarn under when working a head or something that requires stuffing, as I find it makes it tighter! But I am more natural doing yarn over!