I do agree with those comments above. I use both of them but I mostly use YM because YM is the first IM that are very popular here, so > 99% of my friends uses YM for text-based chatting. But... If my friends want to use webcam or voice chat, we can easily switch to Skype.
Skype, of course. It has been better than any other messenger since its release Yahoo Message = Full of Spam People :/