truelies,
If your intention is to call USA to China, without using the PSTN service in China, then you don't even need Google Voice. If the called party is going to own the OBi, then you can easily call them via the OBiTALK number to their home.
To give yourself a second method to call, just in case the OBiTALK service is down, I would suggest getting two free sip2sip accounts, and install one account on each OBi unit. (
https://mdns.sipthor.net/register_sip_account.phtml ) This will give you a fallback method to call them.
It probably takes the same amount of effort to open accounts on sip2sip or Google Voice. You might even consider setting up both sip2sip and GV, giving you three different FREE services to call with.
1) OBiTALK
2) sip2sip
3) GV
You to decide.