The GrandStream doesn't have an FXO port, so you can't connect the unit to your landline if you still have one.
Some ATA's do have an FXO port, but it cannot be used by eg. Asterisk as a gateway to the PSTN.
The Obi supports the XMPP protocol so you can use it to connect to Google Voice, while the two devices you mention probably only support providers using SIP.
I could be wrong, but it's likely that the Obi has more features when it comes to routing calls (dialplan, AA/IVR).