I haven't tried the cone or the Hitachi for that matter but from what I have read, the Hitachi is the vibe for a "clit of steel". It is said to deliver an almost instantaneous orgasm, I prefer a gradual buildup.
I have a Cone and a Hitachi. I love them both! The only thing with the Cone is that the vibrations are diffused even more so than w/ the Hitachi, so you have to kind of get used to that. It is REALLY strong though, easily as strong as the 'low' setting on the Hitachi Magic Wand.
They both have a rumbley type of vibration but nothing will ever beat the Hitachi at power. It is an INTENSE vibrator. The downside is that the Hitachi only has 2 settings versus the Cone's multiple vibration settings. The Hitachi is also limited to the length of it's cord whereas you can move the Cone more than 6 feet away from an outlet.
The hitachi is definitely stronger. But it really depends on if that's really what you're looking for. I, for example, find it way way too and more than enough on its lowest setting. I also think it's too big for frequent using and get annoyed with the chord. That said, it is and has been the favorite vibrator among women for yearrrrrrs.
The cone won an avn best toy type award a few years ago. I hear it's pretty nifty. They're very different toys...