There are a few other Greek derivatives that have both -ean and -ian, like Aristotelian/ean and Epicurian/ean. In the case of Aristotle and Hercules there's a long -e- in the Greek name (Aristoteles).

Epicurean confused me for years (because he's Epikouros) until I learnt it contained an adjective-forming suffix -ai-, Latin -ae-. This is also used in Jacobean.

So I think consistency should make it unstressed -ian or stressed -ean. Personally I don't think I've seen the spelling herculian, so I'd say it with -Ean.