"Question is: When uploading/downloading from the internet, how does Wingate decide which connection to use when both are available? Is there a setting somewhere where I can specify which one is the primary "default" connection? "
For the NAT connection method it is based off the Metric and your mask length in your Windows Route Table. You can override the metric with a windows route commands, or in WinGate by double clicking your adapter within: GateKeeper --> View menu --> Networks --> Network Connections (Bottom centre).
Within most WinGate Servers/Services there is a "Gateway" option where you can set which Gateway will be used. If you have "Any available connection" selected in the Gateway option then it is again based off the metric and the mask length in Windows routing.
When using the Gateway option of a particular Server/Service you will also want to select "Monitor for dead gateways" within the Extended Networking Service of WinGate. Currently this option detects if the internal side of the Gateway is down, but I believe the upcoming major release will monitor by hopping past the gateway.
For more information on routing, please see the following white paper:
http://www.wingate.com/files/routing_paper_letter.pdf