I think it totally depends on what you want to use it for.
The Hitachi is a big old thing, the vibrations can be felt over a large area because the head is so big. You may not be able to use it during intercourse or with a penetrative toy depending on positions. Some people have issues with going numb before being able to orgasm, because it vibrates over such a wide area.
The Eroscillator doesn't vibrate, it whips back and forth. It's much smaller than the Hitachi, so it can slide between 2 people, or between your body and a toy for dual stimulation. However the sensation isn't right for every body to respond to it - some women feel it isn't quite powerful or intense enough. Personally, I love my Eroscillator, and with most toys I'm a 4 vrooms kinda girl. So I definitely like my toys to pack a punch.
It just totally depends on what your body likes and what you plan to use it for. If it's just going to be a solo clit toy or if you want something to use for dual stimulation. If you want something to just vibrate the pants right off you, or if you want more targeted and slightly different sensations.