I am surprised there has not been more discussion on this issue, but allow me to offer my personal observations.
Skype uses a Peer to Peer protocol which means that your computer will be used as part of the Skype network to facilitate others making calls, even when you are not using Skype to make a call.
"3.3 Utilization of Your Computer: Skype Software may utilize the processor and bandwidth of the computer (or other applicable device) You are utilizing, for the limited purpose of facilitating the communication between Skype Software users. Skype will use its commercially reasonable efforts to protect the privacy and integrity of the computer resources (or other applicable device) You are utilizing and of Your communication, however Skype cannot give any warranties in this respect."
This is how a P2P network functions, but my personal preference is to not be part of a P2P network.
However, Skype is a very popular service with many users who are very pleased with it, so I make no judgments there.
** According to the EULA, Skype does not allow access to emergency services (911).
MagicJack is a service which utilizes a USB device to connect your phone through your computer to the internet. So, in essence if you want 24 hour phone availability, your computer must remain on at all times.
You can read more information at this link, which lists some pros and cons of the service.
**MagicJack states that you do have access to emergency services (911), but you should confirm the setup procedure.
The links I have provided should give you a start on researching the services. My personal preferences should not deter you from investigating the services and deciding what is right for you.