Both of these are obviously exchanges and are not designed to store your money on. Look at what happened to btc-e, bitfinex, nice hash, and a bunch of other exchanges which got hacked and the users had to either wait out a period of time for their coins or got just completely ripped of their money. Exchanges do not allow you to access your private keys, therefore they are not ideal wallets for long term storage.

If you're just asking for which one is the superior exchange, then i'd say cryptopia at the moment.

Really you're just comparing two of the smallest and most unreliable exchanges, but since nova is going through changes right now i'd rather trust cryptopia.