This is an old post, but I just recently found an answer, because I just chose between the two models mentioned myself. I had the newer Cary PH-302 and still have the AES PH-1.
The big difference is that the PH-302 (and the 301) have step-up transformers built-in, so they provide gain and loading for a MC cartridge. But, the problem is that ONLY 100 ohms is provided for their MC inputs, which is not correct for many MC cartridges. It was not correct for mine, and I had no opportunity to change it. Plus, every step-up sounds different, so the built-in transformer did not give me the best sound.
The AES PH-1, does not include any input transformer for MC, so an external transformer or head amp is required. Thus its lower price. And, this is a big advantage. It provides the owner with an opportunity to choose the RIGHT transformer for any MC cartridge, or no transfomer at all for a high-output cartridge.
The money I saved with the PH-1 afforded me the opportunity to choose a transformer that matched my cartridge, and in the final comparison, sounded better than the PH-302.