I have never been to the south island. Have snowboarded on Ruapehu the last 2-3 years.
Season runs longer at Ruapehu than it does at Cardrona.
Ruapehu is one mountain, two skifields (plus a private ski field - different pass).
Ruapehu is not a powder mountain. It is all exposed and any powder gets blown off straight away 90% of the time. Expect a lot of ice especially in the back country early on in the season. Best time to go if you are into back country would be spring (late August onwards).
They don't exactly run a great park at either ski field. Maybe a few big jumps and a couple of boxes and rails. Nothing to get excited over.
Plenty of natural features though!
If you are after quality snow go Cardrona.
If you want a long season with awesome back country go Ruapehu.
At Ruapehu you can climb to the summit and board down to the other skifield. Here is a video from the summit down to Turoa Mt Ruapehu, Turoa Summit GoPro HD pole cam - YouTube
Both ski fields on Ruapehu are large so its easy to avoid crowded runs.
Sorry don't know much about Cardrona!