Getting it today.

i would get prophecies first.
Even with factions having a quick levelling system by the time you got quite far into the game most towns were like ghost towns and difficult to get a pug group.
So i imagine with nightfall being slow levelling and not so much a rush the towns for the first month or so will be empty.
Prophecies still had plenty of people playing at peak times even when factions was just released.
Grab prophecies and learn the basics and get nightfall in a month, or when you have got bored with proph

Factions was easier than prophecies to me. I played Factions first. I don't mean mission difficult but overall things like how easy is to make gold to get best armors etc.