F
No manual focus on the F30/F31, only aperture & shutter speed priority of limited settings. Links please to "reliable" reviews that state the F50 is "crap" compared to F30/F31? F40 is how many mm thinner than a F30/F31
? "a lot bigger" "much smaller" ...how many mm's is that?
You want links? Does Google work on your computer?
Working strictly from memory, the F40 is around 23 mm thick, and the F31 is 28 or 29 mm. You can definitely tell the difference when you hold them.
The F50
is crap compared to the F20, F30, F31fd, and F40, but only if you care about photo quality.
Just look for photos from the F50. They're all over the net. Do you really need reviews to tell whether an image is good or bad? Just look at the photos from the F50.......soft like photos from any Canon, Nikon, Panasonic, Sony, Pentax, or Ricoh point and shoot with 8+ MP. Sorry, but if you're talking about photo quality, the F40 is fantastic compared to the competition. Since it has shutter priority, a runner will be OK, even if the ISO automatically adjusts itself to ISO 800.
Is this an organized run that's done within the city? If so, there will be shadows and low light through stretches of the run. High ISO will help you more than IS in these situations, and high ISO is where the F40 will beat any other slim camera. I can take a photo using my F31fd at ISO 800 that are almost as good as my Nikon D50 at the same ISO. Using a high ISO freezes motion better than IS. After all, it doesn't matter if you and your hands are shaking while taking the photo, not if the things around you are also moving.
Or go with the Canon. Just be prepared for bad photos whenever you're in a shady spot near a building, tree-lined streets, etc. You have IS, but when other people around you are moving, it's not going to help.