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
). 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?