The metric is not being used so the most likely cause is that whichever interface is used to make the connection first that interface continues to be used.
You can confirm this theory by having both wireless and wired enabled on initial startup of machines. Then disable wireless test transfer. Then enable and test. This should confirm your current observation.
Now disable wireless, reboot both machines. Retest with wireless disabled and then enabled. I'd venture to guess that now wired will keep being used, even when you enable wireless. If this proves correct then post your findings here and I can provide you two very short batch files that you could run at startup (to delayed enable wireless) and at shutdown to disable wireless.
On a separate inquiry, do both cards have NetBIOS over TCP/IP enabled?
Adding a day later:
@Chad This is an interesting puzzle as I've only had to force metrics on interfaces for specific reasons; otherwise automatic metrics have always worked in directing traffic. You're stating that the metrics actually show wired adapter has a lower metric, so in essence you're seeing that metric isn't being used.
Before digging into the puzzle, here's one more solution (if you don't want to spend any more time on this). You can setup your machines to turn off wireless automatically when wired link state is "connected" and turn on when disconnected. Use BIOS setting if available or a utility software free or paid and probably other options out there as well or a setting on your wireless card driver (Intel ProSET cards have it among others).
One other thing you can try (it doesn't work on my system but some others have reported that it does).. go into "Control Panel\Network and Internet\Network Connections". If your menus aren't visible, press [Alt]. Select "Advanced -> Advanced Settings". Here move the wired connection up in priority. It doesn't tell you to but people have reported that a reboot is needed after that. That could work for you, though YMMV.
Finally if you do want to dig into the puzzle, here is a list to check
Have you tried to manually set the metrics on both cards in both machines? Does it still not use the metrics?
Another possibility is that TCP/IP is not being used.
Is it windows 7, 64 or 32 bit?
If 32 bit, did you by any chance installed NetBEUI protocol?
Brand/model of the router you're using?
If NetBIOS over TCP/IP is set to default, does the router's DHCP server disable it or not support it?
Have you disabled "NetBIOS Over TCP/IP" in either card?
Is the "QoS Packet Scheduler" enabled or disabled?
Are you using IP v6 and v4?
Is windows firewall active and enabled on both machines?
Do the wireless cards have any specific setting in the advanced properties where they may make direct connections to each other once they've been authenticated by the wireless A/P?
Is any bandwidth management set on the lan ports in your router?
Can you provide a print of your full adapter list?
From elevated CMD prompt can you run the following commands and provide their output?
netstat -b (close all unnecessary apps prior to running this to keep the list small)