This has been a raging debate on the Fuji forum for some time now ... it stands right now that the HR mode (high res) is better on the F200 than on the F70, due mainly to the noise reduction required for smaller photosites in the F70.
So the choice to use HR in the F70 is one of risk. Why would one bother? Well, it is possible to get a bit more detail in the image at 10mp. And if the contrast is high enough, even the F70 can do that for you.
Now ... why do you need that extra resolution? Only two reason:
1) You want to crop. This is much less necessary with the F70 because of its magnificent reach, so I doubt that this reason matters much any more.
2) You want to print big. Well, most people target the web or small prints (to 8x10) with their compacts, so this reason has little to support it either.
So ... there actually is no reason to shoot high resolution in the F70 ... but let's pretend that you see a nice contrasty scene and shoot it in high res anyway. Here is an example ... an extremely contrasty tree in the distance with beautiful fine branches.
I made two shots and downsized the 10mp image to 5mp. This gives it quite the advantage over the 5mp image. I then superimposed the mirror image of three sections of this image onto it. You will see in the following image why even if you think you have the right image for 10mp mode ... you may not.
Remember to click on the image above to see the full sized version, which is a 100% crop. So nothing there is hidden (right Dave?)
The actual duplicated areas (5mp is in the yellow) looks essentially identical to the areas beside them. This tells me that there is simply no advantage to HR mode when targeting small prints and the web. In fact, I see little advantage even for large prints ... none actually.
But there is a *huge* disadvantage to the HR image ... the corn stubble has been smeared almost completely away by the noise reduction. Whereas it has survived in the 5mp insets.
So there you have it ... there is *no* reason to shoot an F70EXR in high resolution. I frankly think you F200EXR owners will find pretty much the same story for typical prints, but I understand the rabid fervor that is required to justify owning the lesser feature set ... there has to be *some* reason why one would want to own the 200 instead of the 70 :-)
Here's a rendering of the complete image from the 5mp version. I hit it hard with processing and the stubble turned out really well, which could not have happened from the high res version.