Here’s an interesting peek into the world of free Web-based mobile phone downloads.

It comes from Myxer, a Florida-based website that claims one of the Internet’s largest catalogs of free ringtones, wallpapers, videos, applications and games.

In its inaugural report on the behavior of its 30 million users, Myxer’s Boombox Report compares the traffic it’s getting from users of Apple’s (AAPL) iPhone and Google’s (GOOG) Android devices. From 1 billion downloads over the last three months of 2009, Myxer discovered several trends that may be less significant than they appear:

• Visits from users on the Android operating system grew almost 350% from December 2008 to December 2009, compared to iPhone visits which grew 170%
• Roughly 70% of the ringtone downloads made by Android users, and 48% of downloads initiated by iPhone users, came from the Hip-Hop/R&B genre.
• In total, Myxer delivered seven times more downloads to Android devices than iPhone devices in Q4 2009.

Myxer doesn’t offer an explanation for why Android’s traffic is so much higher than the iPhone’s, but the reasons probably lie in the fact that creating original wallpaper and ringtones is relatively easy on the iPhone. Besides, the Apple’s App Store offers many more games and other diversions than Google’s.

The bulk of Myxer’s traffic, however, comes from neither Android nor iPhone, as the rest of its charts make clear.

But first, here’s the chart that compares the musical tastes of Myxer’s Android and iPhone users:

And here’s the broader picture, reflecting the 1,500 different handset models to which Myxer delivered content in 2009. Note the dominance of Research in Motion’s (RIMM) Blackberry, whose entertainment offerings are even slimmer than Android’s.

You can download a copy of Myxer’s Boombox Report here.

[Follow Philip Elmer-DeWitt on Twitter @philiped]