Well I have Ambiance, aSleep and iChillout (I'm not actually an insomniac, just kind of interested in doing the comparison, and as they were 59p each, it hardly broke the bank.
The winner is...Ambiance, by a country mile. The sounds are better, you can (now) remove and categorise sounds and most importantly of all, it has options to fade the sound gently to silent, so you are not woken up by the sudden cut off of sound. (aSleep has a "fade" too, but it is pretty poorly done - it seems to fade from full to zero in four steps over 4 seconds, as opposed to the slow fade in Ambiance).
iChillout looks pretty with a different picture but some of the sounds are just bad. The "Scottish Highlands" (where I live...) is just a mess of wind-distortion on the microphone. I have no idea why on earth they let that into the app...