There is no comparison between thses two services. I mean, how could you do this. Anybody will say skype. Gtalk is good for text chat, voice call, and for US and Canada calling. But when we talk about skype, through this you can do chat, video calls, voice call, international call any where around the world and many more things. Skype is father of Internet telephony.
To be honest with you I did not even know that there was something call GOOGLE TALK , I feel very comfortable with skype so I would say SKYPE all the way.
Google Talk is no competition to Skype which I think we all know. Google Talk was only launched because of Google's jealousy towards Microsoft with Skype and they obviously failed with promoting it and have now just left it to die a slow death.
I will go for skype because its free of charge and easy to operate and excellent video chat facility.
I would go for skype on google talk because of better voice and video quality. Though I would point out the frequent disconnections on skype for slow internet connections unlike google talk which works fine even on the slow carts.
I have both installed, but I use Skype more.. For business purposes, for talking and chating with my friends..