https://bitcoinarmory.com is Armory's original webpage when it was maintained by Armory Technologies Inc. (ATI) They still control that webpage but it is currently unmaintained.
After ATI stopped maintaining Armory, the open source community picked it up with one of ATI's former employees taking the lead maintainer role. The website for the open source community fork of Armory is https://btcarmory.com.
You should be using https://btcarmory.com because that website is still maintained and the software that is listed there is also maintained.
btcarmory.com is the best one to use.
bitcoinarmory.com has not been updated since version .96 was released.