The fastest method to update a box is with an ISO image. If you don't mind rebuilding the device configuration then do this by all means. Also, if it is a new box that does not have the latest image then use the ISO and skip the "bunny hops" normally needed to upgrade.
The disadvantage of this method is that you lose all the network discovery that the MARS box might have done or been doing. You are essentially starting over. If you haven't been using the box very long then it is a good idea to wipe it clean and start the discovery process over.