all games are not created equal

DJ Hero is a clear win on this one.  I never had fun playing GH by myself; it's just meant to be played with other people.  DJ Hero isn't like that though.  It's actually almost the exact opposite, where group play isn't really worth it. 

DJ Hero is a ton of fun to play and it's one of the only music simulations that actually made me feel like I was controlling the music and not just playing along.  GH is still cool, but just doesn't connect to the music nearly as much as DJ Hero does.

DJ Hero is also pretty challenging in a very new way, which is extremely refreshing given how long guitar controllers have been around. 

Biggest point against DJ Hero though is that it's pretty expensive.  Despite the price point, I still am glad that I bought it and will continue to enjoy it down the road.

Simply put, DJ Hero makes you feel like a DJ.  Beatmania makes you feel like a loser with it's obnoxious difficulty.
I love rhythm games and I love the challenge of something new.  But Beatmania is absolutely relentless with it's difficulty.  You miss 3 or 4 notes in a row and you shoot straight to the red.  In DJ Hero, you don't even fail songs.  You can SUCK, sure, but you don't fail.

Anyway, the gameplay itself from these games are entirely different.  Beatmania's controller is basically a 7-key piano with a turntable.  Obviously, the turntable itself is the most appealing factor, but Beatmania just treats it like a button, which is a real turn off.
This is where DJ Hero shines.  You have three buttons, an effects dial, a cross fader, and a turn-table.  When you scratch on the turntable, you have to do it in rhythm, and on harder difficulties you even have to scratch in the right direction.  It syncs up with the music so well that you really start to feel like you're spinning the music.

Also, the soundtracks for these two appeal to entirely different audiences.  The songs in Beatmania are pretty much copy/pasted from DDR and whatever other Bemani titles.  Lots of techno/orchestra, a bit of rave here and there, but mostly obscure stuff.
DJ Hero sounds like songs you'd actually here in a club.  All songs are mash-ups, meaning it's at least two songs mixed side by side.  Most generally this is a hip-hop or rap song combined with anything else, whether it's an 80's pop song or some rock n' roll tune. 

Anywho, play Beatmania in front of people, you'll feel stressed and feel like a fool.  Play DJ Hero with some people (and get good at it like me :D ) and you'll feel pretty awesome, plus you'll gain respect for DJ's in the clubs.