Hi beeky,
Do you mean theS6500 versus the S5700? The bigger numbers normally come with higher pixel counts which is OK but you have higher weight swinging around your neck and there are more useless features as well. It really all depends on what you are planning to do with the camera. Holiday snaps and kid's parties? I'd just go for the S5700. Are you going to want to take serious pictures of your tank and maybe get creative? Go for the slightly higher quality available with the higher model number. I've heard those Fuji models referred to as "bridge" cameras because they are more versatile than the compacts but not quite like the DSLR's. You'll think I'm an elitist but I figure if you're going to get something that big and bulky why not just get the SLR? Then you don't have to worry about "upgrading" I'd just save up some more and get a Nikon D40 which is a cracking camera £290 at amazon (OK, Ok, I know, it's twice/thrice the price of the Fujis).
TDI-line, same analysis - the Sony compacts take good pictures, and if you only make 5x4 prints or just email the snaps to friends and family no worries, but what do you want to do with it ultimately? If it's for the wife, no problems - give that Casio a look though, she's more likely to want something cute, stylish and convenient I reckon. 8)
Cheers,