Rockstar Games, makers of the extremely popular Grand Theft Auto franchise, are picking up and moving a very lucrative music partnership from Amazon to iTunes, which offers players of GTA IV the opportunity to purchase tracks used in the game’s soundtrack.

The reason for the switch? Apple’s (a AAPL) decision to finally go DRM-free. Rockstar had apparently always wanted to go with the big fish (how could you not, when they command more than 90 percent of the digital music distribution market?), but also wanted users not to be tied down with the music they purchased using it.

The switch will come alongside Rockstar’s upcoming downloadable expansion for GTA IV, which will also introduce a major addition to the game’s existing soundtrack. This is a perfect example of why Apple’s decision to move to a DRM-free music library was the right decision, from the consumer’s standpoint, yes, but also as a sound business decision. Formerly wary prospective partners will now embrace iTunes with open arms.

For us as iTunes customers, this sets a precedent that other companies will hopefully follow, which would lead to more choice, and innovative content purchasing models. With GTA IV, you purchase the music via the in-game “Zit” service, and you can then download the track outside of the game through Amazon’s MP3 store (soon to be iTunes). Video games are an obvious partner, since the interactivity is already there, but movie and television soundtracks could soon follow. I’d also like to see the reverse implemented, whereby you can use any of the tracks from your existing iTunes library in-game.

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Guitar Rock Tour has arrived to challenge Tap Tap Revenge‘s rhythm-action throne. The new arrival touts a bevy of terrific tunes to tap along to, plus the option to play drums or guitar, loads of characters and unlockable awards.

While Guitar Rock Tour may be Rock Band squished on to the iPhone, it’s just as loud and rocks just as hard. Whereas Tap Tap Revenge is free, Guitar Rock Tour costs a few bucks ($9.99) and as such is polished better than a factory fresh Fender Strat.

At the core of any decent music-based game is the soundtrack and, while all of the song in GRT are sound-a-likes of the real thing, these are well performed covers and during a frenetic, in-game jam session the tracks come across as hard-rocking and suitably authentic.

Choose your track in Guitar Rock Tour

There’s a real range of tunes too, most of them classics in their own right, from contemporary tunes like Bloc Party’s Banquet across to classic hits with recognizable riffs such as Message in a Bottlee by The Police. Conversely, Tap Tap Revenge is heavy on excellent, yet obscure, indie and electronic artists spiced with a few chart-topping acts like Kaiser Chiefs and The Offspring.

In essence, the soundtrack for Tap Tap Revenge is more experimental and certainly more edgy. Where Gameloft call on a Blink-182 cover for some cool caché in Guitar Rock Tour, the folks at Tapulous feature girl-kissing chart topper Katy Perry’s latest hit in Tap Tap Revenge.

Play drums too in Guitar Rock Tour

One thing that Guitar Rock Tour implements wonderfully is multiple instruments. For each track you’re able to choose between shredding your way to fame on guitar or beating your way out of obscurity on the drums. In particular, playing the guitar is a joy: with chords executed in a smart way and techniques included for hammer-ons and pull-offs.  As a result, it’s far more tactile-feeling than TTR.

Another feature in Guitar Rock Tour that is absent from Tap Tap Revenge (though not necessarily missed), is a Tour mode. After picking a character and signing a cartoony contract, you and your amateur band take to the road, playing tunes across the U.S. and Canada, and rocking your way to stardom.

Along the way you also pick up various awards, meet some odd characters and unlock new tracks to play live. The story is indeed silly, yet it’s great fun and certainly a compelling way of playing through the game’s selection of 17 tunes.

Stylized clarity in Tap Tap Revenge

In terms of the graphics, the aesthetic in Tap Tap Revenge has a stylized clarity, suiting tons of musical genres, while Guitar Rock Tour is designed with a gritty rock feel — it’s like Guitar Hero meets one of those dingy little bars where the bands jam around Hendrix tunes. Effectively, it’s cliché but it works and screams of console-gaming.

But although Guitar Rock Tour clearly means business, there’s a show-stopping bug that lets the entire game down. On tracks that require more shredding and finger-picking, the backing track falls out of sync with the guitar track. As a result, the game suddenly becomes a lot harder to play and the music ends up sounding like a bad covers band who can’t seem to play in time.

The folks over at Gameloft have made a raucous rocker of a music game, with a stomping soundtrack too, and yet sometimes it just doesn’t work. For now, we’d say hold back from investing until Gameloft patches the problem and releases an updated version of Guitar Rock Tour.

In the mean time, grab Tap Tap Revenge, it may not have all the extras of Guitar Rock Tour, but it’s free, plays very well and includes some fantastic tunes too.

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