I will go with Skype because it will be better and also able to chat with Facebook online friends in Skype.and in yahoo only we can chat with yahoo users.
I only use skype right now, haven't used yahoo messenger in a couple of years. I use google voice for free SMS messaging all the time and phone calls except on weekends when the cell phone is unlimited free.
I use skype because it is the simple for use and does not take up a lot of memory but it all depends on what you're accustomed.
I would recon to use yahoo for personal use and skype for professional use..though voice quality is better in skype compared to yahoo..as per my experience using this tools..try out and then decide!!
I have used both of them. But from my opinion Skype is much better than Yahoo messenger. Now I am using Skype for Video chat and Google talk for instant messages.
I use both of them but mostly i use skype because Skype will provide all like in yahoo and will be more efficient than Yahoo messenger.