Skype because it is offering call services free worldwide but google offering for just Canada and US free.
I usually use Skype for better conversation to your client especially on discussing information about sharing files. So it's more reliable in business matter.
I dont know about the Google talk or any other similar tool, because all the people I know are using Skype and its no point for me to switch to other. Its the same with Facebook, why is it most popular? Why is DPforum the most popular?