I have heard really good things about both of those. I don't think you could go wrong with either. But for what it's worth, I just put an m4 64gb in my macbook unibody (2010) and it works great!
The two drives you mentioned are the two that I heard the most good things about in regards to mac's and osx.
Really the only difference (correct me if I'm wrong) is that the 320 is sata II (3gbps) and the m4 is sata III (6 gbps).