Last time I looked at the trials they were full versions, just time limited. I think HEM's trial is 30 days and PT3's is 60 days. You'll know within 30 days which you'll prefer. HEM also offers a full money-back-guarantee if you buy it and change your mind. Not sure if PT3 does or not.
I started with PT3 while it was in beta and through its release, but HEM won me over. At the time though I was almost exclusively a cash player and HEM initially had poor tourney support compared to PT3. Since then however HEM added tourney support and are actively working on shifting more resources towards additional tourney functionality, recently polling their users on what they want to see added for tourneys. Today I play mostly tourneys and almost no cash, and I'd still pick HEM over PT3.
One reason I prefer HEM is their support. Their support staff is on top of the forums, and they are very quick to push fixes and other updates out. With the exception of the Rush HUD which PT3 beat them to, they pretty much stay ahead of PT3 on support, updates, and new features.
That said, PT3 is a good product and you can't really go wrong with either one. Both will improve your game a lot of you apply them properly, particularly with respect to finding your own leaks.