I believe the "Lower 49" option is for the download version only. To have the choice between download, microSD and DVD, you'd purchase the North America NT...which Garmin has for $99 -- not $59.
Anyhow, the download vesion is restricted to one device only; this is also the case for the DVD version. With the DVD, you can create and manage routes/waypoints/etc as well as save on multiple SD cards if you wish (for spares or for dedicated purposes), but I don't know if that's the same for the downloaded version. You can also purchase the periodic updates. The microSD version can be moved with multiple devices, but it doesn't allow the same map management features, and you cannot buy updates for it. But, if I'm correct, you don't have the option of DVD or microSD with the Lower 49 version.