That Amazon S3 is cheaper than traditional dedicated servers is a myth that's been spreading for a while now.
You can argue all you want about how reliable and convenient it is. But it's not cheaper. In fact, it can be very expensive especially as you consume more bandwidth and storage.
There are certainly many cases for using S3 even though it is more expensive. There is nothing wrong with that.
It's just that I have met way too many people who after describing my situation, one of the first things they say is "have you tried S3? shouldn't it be cheaper?"
That depends on how steady your demand is. If once a year you spike to 1000x you base load S3 would be cheaper for you.
If your going to keep 3 copy's of 5+ TB over 2 data centers then S3's costs for storage is fairly reasonable but a little pricey. But if your quickly growing you need to compare the month to month cost of data storage vs the cost of new hardware.
You can argue all you want about how reliable and convenient it is. But it's not cheaper. In fact, it can be very expensive especially as you consume more bandwidth and storage.