* More people that I'm in contact with have Skype installed vs. have Google Voice and Video installed
* I use Adium as my IM client as I find it easier to use than Google+ in browser chat. To avoid double notifications, I keep chat closed 'in browser', which adds friction when starting a hangout
* Skype makes the call much faster than Google+ hangouts. In hangouts, the call usually times out but if I leave the window open, the other party will eventually join
On the plus side:
* Google+ hangouts call quality is usually much better
* Skype can't do multi-user video
* I find the 'in call' Google+ hangouts interface much more intuitive.