Honestly, last year was my first for either and I thought Howl-o-Scream was just as enjoyable.
Howl-o-Scream wins:
Less crowded
Shorter lines
Able to easily go inside places to eat
Themed bars/eateries
Cost
In-your-face scareactors
Scare zones - much more interactive, creepy, scary vibe, as opposed to the basically photo opp zones in HHN
Enter houses spaced out in groups
HHN wins:
Overall set design/quality of sets
Number of Houses
Themed food (if you can find it)
Ties:
Interesting theme of houses
Shows - the Jack the Ripper show & the Pyro show were both really good!