Hi snr345,
May I offer a warm welcome to the CameraLabs forums.
I've moved your post to the "Buying a new camera" forum (but left a shadow in the "Travel photography" forum so you can still find it) because, while the new camera is primarily for your Aussie trip, I think you will get the best response to your questions here. Hope you don't mind.
I'm sure you have already read Gordon's reviews of the Stylus 790 and PowerShot SD870 (
here and
here). It would seem from those reviews that unless you really need the extra environmental sealing of the Olympus you are giving up quite a lot in terms of IQ. If you are only interested in small prints (or viewing from distance on a modestly sized screen) that might not be an issue. If it is then maybe a really good camera case for the Canon might help keep the sand away from the camera when you aren't shooting?
Bob.