When I last time bought a Canon Digital IXUS compact camera, there were basically two series available:
"Ultra-compact"
Digital IXUS 65 ... 70 ... 75 ... 85 IS ... (Europe)
Powershot SD630 ... SD1000 ... SD750 ... SD770 IS ... (US)
IXY Digital 80 ... 10 ... 90 ... 25 IS ... (Japan)
This series was a bit smaller, more lightweight and cheaper, but the image quality was lower, i.e. the images looked like a noisy picture that has been smoothed digitally, like images from many modern mobile phones.
"Compact"
Digital IXUS 300 ... 400 ... 500 ... 850 IS ... (Europe)
Powershot S300 ... S400 ... S500 ... SD800 IS ... (US)
IXY Digital 300 ... 400 ... 500 ... 900 IS ... (Japan)
This series was bigger, (earlier models) used CF cards instead of SD, had somewhat more features and provided much better image quality.
At least in Europe they were quite easy to tell apart, not only by looking at the size of the camera, but also because the ultra-compact ones has 2-digit model numbers.
In their current models, I don't see this distinction anymore. Does that mean it doesn't exist anymore and I can expect the same image quality from any of them?