Although really, early first-season Kirk is the exact same character as Pike -- a serious, driven, brooding, lonely starship commander. He only evolved into a different character as Shatner's performance influenced the role and (probably) as the writers came under network pressure to make Kirk a more conventional '60s action hero.
I agree. Hunter and Shatner's relative performances boil down to different takes on the Horatio Hornblower inspired writing of the captain character. To fully appreciate it I would really recommend reading one of the Hornblower books (C.S. Forester author). I did because of Roddenberry's constant citing it as a source and boy, he wasn't kidding. The influence is palpable.
Also it was common practice for actors to shoot pilots for show's they felt wouldn't get picked up. It was easy money. And, at the time, Jeffrey Hunter was a "serious" film star where as science fiction (especially on TV) was kiddie fare and not highly regarded. That it is why Lloyd Bridges rejected the lead in Star Trek outright and why Jack Lord placed such a high-price (50% stake) on his services (knowing full well Desilu couldn't agree to his terms).