Here's the official scoop from Garmin.
I recommend the DVD option. Buy it to start with. When you want to upgrade maps, purchase
https://buy.garmin.com/shop/shop.do?cID=256&pID=33379nüMaps Lifetime North America. Although it appears to be a nüvi product, it works with many of the newer handheld units.
DVD/CD: Maps purchased in this format will require a computer to use but will allow you to plan trips on your computer and transfer waypoints, routes and tracks to your GPS device. The installation process will install the mapping and MapSource to your computer. MapSource can then be used to send maps to either your device or a datacard. Please note that most mapping products in this format are unlocked and can only be used on one device. City Navigator maps in this format can be updated by purchasing a mapping update, when available.
Pre-programmed datacards: Pre-programmed datacards are immediately ready to use. Simply unpack them and insert them into your compatible GPS device. These maps do not need to be unlocked to your device which allows you to move the card to multiple units if desired, however you will not have access to the mapping on your computer. City Navigator maps in this format would need to be purchased separately when a new version is released.
Downloadable Content: This content can be purchased and downloaded directly from the Garmin website. This is the quickest method of getting mapping for your next trip. Downloaded maps can be sent to either your GPS unit or a blank datacard. All maps in this format are unlocked to your device and can only be used on the device that they were unlocked to. Similar to the pre-programmed datacards, this mapping is not available on your computer for use in planning routes. City Navigator maps in this format would need to be purchased separately when a new version is released.