I've built some basic prod rooms lower in the vault to rank them up, what gives more xp, a maxed out power generator, diner and water treatment or the level 1 versions of the nuclear power station, greenhouse or water purification?
You won't want to upgrade the production rooms where you level-up dwellers. Every time you upgrade a room, you upgrade the disasters that happen in that room.
Storage capacity is the only significant advantage of Greenhouse, Purification and Reactor over basic Diner, Filtration and Power.