2. 500 megapixel => 10R, 400 megapixel =>8R
if you need to crop photo and enlarge it, 500 will
be better
3. ixus 500 => light weight and small, easy to carry,
A80 => more manual control function
4. ixus 500 => no hot shoe for flash
I am using ixus 400, color, exposure and sharpness are very good. But the flash is not enough and the rechargeable battery can only last for 1.5 hr.
I would prefer ixus 500 rather than A80.
If you are a beginner, I think ixus 500 will most fit you.
If you are not, I think you'd buy a professional model rather than A80
photo taken with ixus 400, crop without any correction, resized from 1M to 56k
I wish to remind you that don't just look at the price
Please also see the accessories too.
Besides the DC, some come with 256M CF card, some come with 16M CF card. Some come with an extra battery or tripod. Therefore the price will not be the same. So be careful to check it first.
I recommend you to buy one more battery and one more CF card for backup / exchange