Thanks for the feedback everyone. It sounds like I should just work out which snake I prefer, but garter or corn would be the easiest to care for. I still need to get the go ahead from the landlord, so I'll put a bit more thought into which I'd prefer. I think I'm leaning towards garter snake, just because of the easier feeding, although corn snakes do still appeal and seem easier to get hold of in various designs than garter snakes
Also, thanks for the recommendation of getting a younger snake so I can watch it grow, hadn't really thought of it that way and I think that's probably a good call.