I don"t think there is a great deal between them. I"ve used PT for years, since the original version, and have never come across anything that I need that can"t be achieved.
I had a brief look at HEM some time ago, but couldn"t see anything that it could do for me that wasn"t already available in PT3.
Basically both are the same product underneath in that they pull hand histories into a database and then extract that data in various forms to assist with analysis, HUD, reports, etc...... Creation of custom reports in PT3 is particularly easy - I can look at my STT performance and results in just about any way I choose with PT3.