*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Unless you specifically want HTC Sense features, I strongly suggest buying the HTC One Google Edition from the Google Play Store. It costs the same as the unlocked version and is compatible with AT&T's LTE, whereas the unlocked version is not. The Google Edition comes with stock Android, which will enable it to get prompt updates, and is fully unlocked.
Purchasing the phone from Amazon, is essentially just buying it from an authorized AT&T dealer. You can buy it for the subsidized price and sign up for a 2-year service contract, or pay full retail. But its still SIM locked to AT&T.
If you need SIM unlocked, buy from HTC or Google Play Store, as previously suggested. Or from a 3rd party vendor that specialized in unlocked phones.