The OBi100 and OBi110 have a PHONE connection for a telephone / cordless phone base station and an Ethernet port to connect the devices to you Internet router. For making and receiving calls to and from a VoIP service (like Google Voice and Callcentric), they are identical. The only difference is that that OBi100 does not have a telco LINE port.
The OBi110 has an additonal connection for a telephone line. With this connection you can bridge calls from a VoIP service like Google Voice or any other VoIP provider to the service providing dial tone on the analog line. If you do not have a telco service at your home (or are going to get rid of it), an want to make all your calls over your broadband Internet connection with Google Voice or some other VoIP provider, then the OBi100 is the way to go.
Here is a comparison chart from Obihai: http://www.obihai.com/matrix.html