I am 100% going with skype. Skype is much better against yahoo in voice quality, sharing files, performance etc.
When I was using Instant Messaging, I was using Yahoo, AIM, Skype, Windows Live, G-Talk, & ICQ.....(can we say obsessed?) but......I don't use any of them anymore. When I did use them, I found them all to be useful.
I prefer Skype personally. I've also started using Wire.com which was founded by one of Skype's co-founders.
Uh I think skype, yahoo is kinda dead nowadays, I see they have some users yet but.. I don't know, skype is certainly more popular!
Definitely Skype. Although, Verizon just bought Yahoo assets so that could change down the road. Have to wait and see.