Landlords controlling the electric tariff can make a huge difference to what you pay but 9p a unit doesn’t sound to bad, can they just change this when ever they want or is the rate written into your lease?
Watch out for other charges as well like any service charges that they may add on top of your rent and also check to find out how you stand with things like repairs to roads around the estate any upgrades to electric etc that you may be asked to contribute towards.Like John I would be inclined to take the larger of the two units but you will have a better Idea of what space you need both now and in the future yourself.