Last monday I had the opportunity with friends to do some measurements of spectrum on various 2.4Ghz modules.
The initial intention was to measure the difference between 2.4Ghz antennas, but the environment (in a shop) reveals that it was impossible.
So, no results about the antenna efficiency, but we could observe the spectrum of various transmitter. Thanks to flash-rc who makes this possible to test different transmitter from various vendors.
First, here is the spectrum print for the Xbee module that is used in some home made RC. You can see that the spectrum is not very sharp around the 2.46Ghz channel, and there are many harmonic around.
Then we tried Assan module. Very interesting. Its Spectrum is very sharp and neat. It uses DHSS technology with two channels simultaneously as Spectrum. It looks clean.
Then we tried several transmitter together to compare their Spectrum. The picture below shows Assan, Spektrum and Xbee. The Xbee transmit only on one single channel at a time.
Finally, we tried two Futaba transmitters simultaneously. On the picture you can see that they use the entire spectrum 2.4Ghz. They use the FHSS technology. Don't be afraid by this picture, the spectrum here is remanent and it does not mean that the radio continuously emit on all these channels at the same time.
In conclusion of all this, if I have to buy 2.4Ghz module, I would seriously investigate Assan as their Spectrum looks very clean, which make feel confident in it. But of course it is just one aspect. In addition, it looks that the DHSS technology is more suited to RC because it seems to be more robust against jamming. DHSS techology is based on CDMA, made for the Army for Missile guidance and comms. More info on DSS techno here. FHSS primarly purpose is communication crypting for army to prevent comms hijacking.  But it looks that FHSS used by Futaba is not exactly same as FHSS used by the army. Subject is complex. There is an interesting thread about this on rcgroups