Thanks for all of the great feedback guys - can always count on this community.
My contract is up in August, so I have a bit of time to think about it. I'll probably weigh up the price difference of paying a bit extra for a new phone on a contract here in Aus (Telstra), vs paying outright for the Nexus version.
RE the N4 - my fiance has a N4 but I'm already smashing my phones capacity on my S2. I like the idea of being able to sneak in a micro SD, which is why I'm leaning towards the S4.
I will have to do some more reading about those phablets though.... I could use a bigger screen as I hardly use my phone for actual calls.