Now that Microsoft is putting major development and marketing money behind Skype and integrating it with Windows, I wish Yahoo the best of luck.
I do not use either because IM and over-the-web telephony are source of wasted time. Sure, I'm open to use any of these and talk to anyone for unlimited time as long as they pay my hourly rate for live chatting. For family and friends, landline and mobile phones are easier and faster ways to be in touch with me, besides email and regular text chatting.
I use both messengers as I keep a lot of different contacts on both but Skype video call/webcam is much clearer and better than Yahoo video call/webcam.
Depends for what. I think is Skype better for communication,but the Yahoo also have pretty nice features. Skype will never change,and it will stay my first choice for communication with people.