In general .wav, since MP3's use lossy compression. WAV files are also generally a lot larger.
But it all depends on the sample rate and bit depth for the WAV vs. the bit-rate for the MP3. An MP3 at 192kbps has the potential to be of much better quality than an 8-bit/8kHz WAV. It's like asking which is heavier, steel or wood... it kinda depends on how much of each you have.
