I am not sure default behaviour for specific WLAN cards, but I know that many allow you to choose preferred band, or at least specify which to use. In my environment with many types of clients, I find most seem to prefer B/G. This may be partially due to most cards only being b/g, but I have seen several cards with default settings prefer G over A when both were acceptable signal strength.
If you want to check if you can set a preference for your NIC, I would check your client utility (ProSet for intel, ADU for Cisco), or if you are using Windows zero config, you can do properties on your NIC and see the options offered by the NIC driver. My inlet and broadcom cards let me disable any of the specific wireless bands.
In my opinion, the confusion of multiple SSIDs would not be worth the benefit.
Eric
