Both Google Voice and most of the SIP-based, pure-play VoIP providers are using various combinations of IP-based carriers to pass audio during calls. There are many points of failure or degradation along the path from one end to the other, over the public internet. It's difficult to try to generalize that one end-to-end call via one ITSP will always be better than another ITSP's calls. Each ITSP can choose to route each call via one or more carriers, and those carriers can subsequently route their calls out via other carriers, in series. On top of that, you have the (often more significant) variations in your home network connection to your ISP. Regardless of the length of the call, all ITSPs are susceptible to sound quality issues at some point.
Some ITSPs advertise that they only use "premium" routes, for a "premium" rate. Certain carriers that provide higher-cost service actually do control more of the call path, and can indeed offer higher-quality connections. For example, Comcast is one of the largest owners and operators of IP networks in the country. They can use their own, dedicated private network for most of the call path, using traffic routing and QoS to prevent disruption from other network traffic. This is one example of "you get what you pay for".
You can take the ITSPs out of the picture, and simulate end-to-end SIP VoIP connections, to objectively measure your own home network and ISP's reliability for VoIP calls, by using the VoIP simulation test provided by Visualware:
http://myspeed.visualware.com/index.phpClick on your country on the map to get started, and select the G.711 CODEC test to several different endpoints. Repeat the tests, at several times during the day and during the week, to get a broader set of measurements. Any test result below a MOS of 4.0 suggests that you have a problem with your broadband service.