This. If you want back heat you'll have to put a back panel onto the rack, which isn't a big deal.
Also a warning for racks with the tubs that go in longways like yours, sometimes the snakes can push hard enough in the middle of the long side to escape.
What animal are you housing in that rack? If you want bp adults in it, I would say front to back. If you are keeping anything else back heat would be ok.