Pre-wire for the fan with two switches(1 for light and 1 for fan). Its so much easier to do now and you will have the option of doing a chandelier or fan!

I thought they were interchangeable?

They are interchangable and it can be done that way but if you just wire for a chandelier you won't have two seperate switches (1 for the light and 1 for the fan). There would only be one switch which would control the chandelier. If you switched to a ceiling fan afterward because you only have one switch, which would either control the fan or the light (not both), that would leave you having to purchase a fan with a remote control. If you pre-wire for a fan with two switches to begin with all of your options are covered up front. Hope I'm making sense! :)

I thought a fan box is heavier duty too so it can support the weight of a fan with a light, which can be heavier than a just a light.