Take two photos of the same object, one in each mode, and view them in photoshop at 200% or so, you'll likely notice more "compression" in the photo taken at "normal." (you'll see artifacts, pixels that seem blurry or out of place, odd colors, especially on edges where there's contrast, and large areas of even color might be smoothed out. If you don't see the difference, look in one of the RGB channels, it's usually more obvious.
here are some examples, you can download them and compare them up close:
http://www.dpreview.com/reviews/canong6/page12.asp
The G6 seems to have pretty good compression compared to some cameras, and if you're making 4x6 prints from 7mp images or posting them on the web or just looking at them on-screen, and you want to save hard drive and card space, and you're more interested in quantity than quality, you'll probably be OK in "normal," but be warned that in some photos it might be noticeable and if you're planning on having any large prints made, or color separations made for 4-color offset printing, your photos are not going to look the best they can. But for that matter, "Superfine" isn't the best, it's still a tiny bit compressed, "RAW" is the only uncompressed format.
Why not split the difference and at least shoot in "Fine"?