For me playing solo, the question is 2 heroes or 3? Aside from the time and management overhead, the third hero is essentially 'free' in terms of quest difficulty for everything except spawning rules because 2 and 3 hero parties are lumped together in the same bucket.
Perhaps I'll try 3 for an especially difficult quest or as a way to experience more heroes faster.
I did a solo play with 3 heroes and it seems the most beneficial. If it's your first time, I would definitely recommend it.