Yah I was just crusing around this PSU site to find out the exact same thing. The only thing I see right now is the difference in the casing colout P being lighter blue where AX is a dark metalic. Feature for feature they seem the same. Some one said the AX is newer than the P. Other than that *shrug*
also be aware of the difference between the ATX12V v1.3 and v2.0 versions of both the Noisetaker P & AX lines http://www.enermax.com.tw/products.php (Taiwan is no longer listing the Noisetaker v1.3s) http://www.maxpoint.com/ (but Maxpoint (Enermax USA) still is)
the Coolergiant Line is all ATX12V v2.0 in both P & AX