| Both can be made to work for atmospheres. I've used Absynth quite a bit to create textures and BGs. I definitely prefer to use it as an effect for real sound rather than as a signal generator.
As for vehicles--you can make them work (lots of control over pitch and filter curves) but you'll need to start with synth signals. It wouldn't be my first or only approach but I'm not much of a synth man myself. Someone who knows how to use those signals well could do some pretty cool stuff with the programs.
If you know the basics of a synth (signal to filter to effect to master) you'll be up and running Absynth in under 5 minutes (but it will take you a while to get to the bottom of its feature set). Reaktor is much more powerful but will take longer to get up and running.
Also, Absynth is buggy as f*ck when working within ProTools (Absynth 4 + PT 7.4). Not sure about he native version or inside other DAWs.
-Dave |