I've got a 2215 as well and I use the SD slot for memory and the CF slot for WiFi.
The trend in devices seems to be getting away from CF slots. The 2215 is the only iPAQ with a CF slot. Also, the most popular uses for CF expansion are WiFi, Bluetooth and GPS. Most of the new devices have at least one of these built in and many new devices have more than one.
So if you went with CF memory and SD expansion, you may end up getting into trouble with your next device. Say it's already got WiFi and BT built-in. You would then have a useless CF memory card and an equally useless SD WiFi card.
Of course this all assumes you're going to be upgrading your device in the next year or two. Beyond that, who knows...