Neither scrypt nor sha256.

As it' s said in their whitepaper CryptoNight is "a new memory-bound algorithm for the proof-of-work pricing function. It relies on random access to a slow memory and emphasizes latency dependence. As opposed to scrypt every new block (64 bytes in length) depends on all the previous blocks. As a result a hypothetical "memory-saver" should increase his calculation speed exponentially".
More info you can get by yourself here.

So scrypt gives us ASIC-resistance, but this algo is much better at this field.