Which is more secure wi-fi or aircard?

An aircard by default uses the encryption of the cell phone system without the user doing anything.

The WiFi matches its encryption to the router it connects to. If the WI-Fi connection has no passWord it is non-encrypted and very insecure, if the Wi-FI uses a passWord it is using at least WEP perhaps WPA to encrypt the signal. Most public WiFi connections don't use a passWord and so are inherently insecure.