3g uses more than double power of 2g for me. Except when idle, power consumption is low enough for both in that case.

In the case of streaming audio, 2g uses about half of 3g. 128kbit/ shoutcast stream.

In the case of downloading stuff, 3g uses alot less per byte transfered. I tend to switch to eg for downloading few hundred megs with gpodder, and switch back to 2g after that.

In the case of lots of IM/IRC, 3g uses about 5 times more power than 2g for me. 3g is really bad at low-rate but frequent transmission.

And as always, it depends on your network's configuration. Signal power plays a role, but for the lowrate frequent transfers the network settings have much bigger impact.