I have been using a Mag 250 for about a year and have just set up a 254 for a friend. To make it clear to you, the difference between the Mag boxes is speed when navigating around the menus, and nothing more (as far as I am aware). I haven't noticed much difference personally, although yes, it is more responsive / faster when shifting up and down that channels, and selecting a channel.
Out of interest, why are you moving away from the Zgemma? I ask because I am thinking for moving away from my 250 and getting a Zgemma.