They aren't exactly the same thing.
The DWL-G810 is a wireless bridge. It 'converts your Ethernet-enabled device into an 802.11g wireless device.' This is not normally used for computers, but rather printers, storage, etc. You plug your ethernet cable into it. It also requires a separate power adapter.
The airport card installs inside the mini and allows it to connect to a wireless access point.
There are usb devices that do this too, but they use a usb port.
Get the Airport Extreme card
PS. All 802.11g devices are generally the same speed. You won't get the full 108MB out of the dlink unless you get the correct router. Internet doesn't travel 54mb anyway.