Skype seems to be more popular and more usable. Honestly I wish hangouts were more used. I tried using hangouts with my pair and our other friends, and it was absolutely FUN. Like, we can take pictures of each other...without the use of a camera and not having to "be there". It's like we're actually hanging out irl, and that's what I love about it the most.
Best of all, it doesn't clog my CPU, while skype does when I have 10 active IMs. ugh ;;