With the DVD version, it includes an unlock code so it will work with only one Garmin device. And won't work if you load it onto any other devices (without hacking...).
With the SD card version, it is locked to the card. So it will work in any Garmin you put the card in, but won't work if you copy it onto another card. So you can't make a backup of it (at least without hacking...).
And you can't combine the SD card with other maps on the same card, eg contours or OSM. But with the Etrex 30 you could put the contours/OSM maps in the Etrex internal memory, plus use City Nav on the card.
Also, with the DVD version, you are able to install the maps in MapSource or BaseCamp, so you plot routes etc on your PC. You can't do this with the SD card version. But BaseCamp will read the maps from the SD card while the Etrex is plugged into the PC.