I'd like to hear why as well. This was certainly true during HEM's early days, but I don't believe it's the case for the last year or so. Unless PT3 has added a lot more since I last used it (I left PT3 for HEM when HEM was released).
They have done a TON of work since you left. 90% of their new development in the last year has been on tournament related functionality. Everything from simple stuff like displaying stacks in BB or M on your HUD to extensive tagging of tournaments so you can analyze results by all sorts of tournament types. Turbos vs regular, rebuy tournies, satelites, KO's etc. (Or whatever combo of criteria you want to use, it's user definable)
Auto-imports stars audit reports now as well so you can get all your R/A info auto-loaded as well as get old tourney buyin/winnings info if you have the tournies in the DB but never brought in the tourny summaries. (This can save you a TON of work if you want to get all your $$$-related items for past history.)
Also, runs handgrabbing and HUD for
bodog tournies, which HEM (unless recently changed) does not. HEM only supports Cash.
Support for grabbing bounty prizes on FTP and elsewhere where it is available so you don't have to do it yourself.
I'm sure there is much more, that's just off the top of my head. Also, I read te HEM forums extensively during this period (since I am a volunteer tester for PT's new releases - I was curious what HEM did/didn't do.) If you read the Tournament improvements thread there, even the posters on the HEM site itself make it clear that they feel they are well behind PT3 for tournaments.
I speak for MTT's specifically. For you SNG grinders, there
might be more of an argument as to which is better if you are using all the extra purchase addins on HEM.
If you really want to know, just go to the PT forums and check out the release notes threads. And FWIW, I know the stuff works as a lot of things got done to my specs or a variation on what I requested, as I was the contributor that played MTT most/ cared most about MTT stuff.