Have just played a little...
(Edit) With TuneBlade, by defaultyour initial option is to only be able to play on airplay enabled devices. Once you start streaming to your speakers, audio "shifts" from your PC to the destination speakers (that's a bit annoying). If you want to play on both your PC and your airplay enabled speakers, you need to install TuneAero on your PC so that it becomes an airplay enabled endpoint.
Airflow seems to have much more control, allowing you to send concurrently to your PC and airplay enabled speakers, select source, etc etc.. There appears to be much more control afforded to the user. I still need to give airserver a shot to compare....