I've read a few articles on Image Compression. That's where you specify if your camera is taking Superfine, Fine, or Normal quality shots. Different brands (and years) of cameras have different names for this - but it boils down to the same thing.
Most of these articles suggest that, if you have the room, you should *always* set your camera to the highest quality and highest Megapixels. However, most of these articles are at least three years old - which is a very long time ago in terms of technological progression.
Not a lot of them include examples either - which would still be outdated, but at least would give you an idea of what to expect.
Of the few that do give examples, they tend to magnify them to the point where the difference is actually visible (300 or 400%) which doesn't strike me as particularly 'Real World'.
So without further ado....
High Quality above, Lower Quality below.
As you can (probably?) see, the top image is slightly better. The edges of the leaves are sharper, and there is less compression artifacts around the web (which somewhat exaggerated due to motion blur). There is also a bit more dynamic colour range in the top photo. It's also twice(!) the file size.
Is it worth it? This is where most other sites would say 'Yes, if you're going to print it - memory cards are so cheap - go with highest quality'. A few sites will (paradoxically) suggest that you switch to lower quality *when* you start running low on space i.e. when it's already too late. In my mind, having (at least) twice as many pictures for a rather imperceptible change *is* a very good tradeoff - and much better started early.
There's also a psychological advantage to that as well. If you assume that 'Highest Setting' means 'Best Shot', you probably won't take a second picture, even if you probably should. If you have twice as many pictures remaining, it's a lot easier (mentally) to spare the room for a 'Backup Shot' - even if it's the difference between having 2000 pictures remaining or 1000. It's worth noting that some of the pictures were up to 4x smaller on the lower compression mode of the T2i - it varies a lot between cameras and shot content.
When taking the pictures for this article a rather high number 'messed up'. Either they weren't in focus - The A3000 shots (not shown) had half of either pair blurry - so weren't worth putting up. They looked fine on the camera preview. A significant number of the SLR shots had a different area in focus, or motion blur. In not a few shots, the smaller file size shot was noticeably better than the larger. In other words - it was much more likely for *something else* to go wrong, than for compression to mangle the image.
You mileage may vary, especially with an older camera. It's worth looking into though. Memory cards and Hard Drive space *are* relatively plentiful and cheap - but still finite.