"Hero Powers are often weaker than God Powers.
For a balanced game, we recommend only playing
Hero Powers against other Hero Powers.
Hero Powers can also be used to balance games
between players of unequal skill, whereby more
experienced players are encouraged to select a
Hero Power for themselves and a God Power for
Because Hero powers are often weaker than God powers, including just one when drafting powers is usually pointless, especially in Arena where you aren't trying to handicap either player. They are unlikely to provide a balanced match-up with any of the Gods on offer, so often the inclusion of just one Hero means you have one less viable choice for powers. If I select a Hero power and a God power for my rival to choose from, they will almost always pick the God power and leave me with a weaker ability. There are exceptions, of course, but generally speaking, I'd prefer a God power when the other choice is a mere Hero.
I suggest when mixing these powers, if there are any Heroes at all, there should be a minimum of two Heroes. That way, whoever chooses the match-up can suggest a balanced Hero-versus-Hero game if they want. Otherwise, when there is just one Hero, usually that means you have one less viable choice for the match-up.
I don't have the data to support Heroes being weaker. I am strictly going by the quote from the rulebook and my own personal impressions. If I'm wrong, that'd be interesting to learn! I think my suggestion would still be worth considering in that case anyway because of the perceived power imbalance and because some might find it more fun to do Hero Vs. Hero, and it's rarely an option for mixed selection.