all games are not created equal

As a serious music-game lover, I adore both of these games.  Frequency is actually the first music-based game I've ever played, period.  I was quick to snatch up this game and it's sequel, Amplitude, when I heard it was coming out.
Technically speaking, Amplitude is an improved version of Frequency.  It has 3D avatars instead of a 2D cut n' paste pallet that Frequency uses.  Also, the backgrounds are more detailed, and the soundtrack is more diverse than Frequency's techno and hip-hop heavy based track list.
However, being a music game lover, I love challenge, and I find Frequency much more challenging than Amplitude.  Back in the day, a special disk that came with the PS2 network adapter allowed you to play Frequency online as well. 
I love techno, I love challenge, I love Frequency.

You may not have heard of either of these games, and that's a serious downer on your part. These two games (with Amplitude being the sequel) are rythm-based, requiring you to hit notes on the track, using 3 buttons. This may sound familiar, and if it does, it's because these games were made by the same company that made Guitar Hero . One of the differences between these and Guitar Hero is that you play many different instruments, ranging from Guitar, Synthesizer, Vocals,  Drums, Bass, and others.

Frequency has you in a large pipe-like construction, wailing out on the controller to great songs by a multitude of bands, with powerups along the way to help you out. Amplitude is basically that - only, You're coasting on a mostly flat surface, and there is a few powerups added, such as the Slow-mo, which, of course, slows everything down allowing you to hit notes easier. There is a cheat to return to pipe-mode, but it's just not the same. The original Frequency pipe was better.

But Amplitude more than makes up for that. The better selection of songs, the harder difficulties (you'll get there eventually), the online multiplayer (sadly taken offline forever), and the ability to make and share tracks you made for certain songs.

Amplitude shows Frequency up in more ways than one, however both are definately a smart choice.

P.S. I know a lot of this is confusing, so I present to you:

http://www.videogamecritic.net/images/ps2/frequency.jpg

http://image.com.com/gamespot/images/2003/ps2/amplitude/0326/0002.jpg