The Sennheiser microphone scores more highly and is in a better position than the dangling Icemat mic (which makes it susceptible to noise from the movement of clothing). Recording quality with the microphone was well rounded and the noise cancellation makes a real difference. With the microphone in its lowest position, beneath the mouth of the speaker, the results are really rather good whereas the Icemat tends to sound very thin and biased towards the high frequencies.
The Sennheiser comes with a USB sound card adapter whilst Icemat offer this as an option. I could find no information on the sound chip used by Sennheiser as Windows recognises it as a generic USB audio device. No drivers should be required, although one of the three machines I used to test them on didn't install the drivers automatically and whilst this is no doubt a problem specific to that machine it would be sensible to provide the drivers as backup. The noise level of the USB device was below that of my on-board Avance AC97 chip set but was still poor. Itâ€™s a very tidy solution if your machine (ahem, Apple iBooks, ahem) doesn't have a microphone input and it does handle all the sample rates that you'd need, including stereo even though the microphone on the headset is mono. However, if you intend to be doing any serious recording, it's not a replacement for a decent sound card.
The Icemat usb sound card is based around the C-Media chipset with controls for volume and speaker or microphone mute. The device is capable of basic environmental effects including EAX 2.0 with 16 hardware voices. As a basic game playing card, it's certainly passable and offers better value for money than the Sennheiser. Overclockers are selling it here for Â£29.32 including VAT.
Both are quite expensive for what you get and this is because of the USB sound cards. Deciding between the two must come down to the kind of applications you need a headset for. If voice-over-IP is your thing, the Sennheiser PC155s are a neater and more convenient solution that are more comfortable to wear and easy to move around in. If music and gaming is what you primarily need a headset for, with only occasional use of a microphone, the Black Siberias, even with the separately available sound card, offer better value for money.