The short answer is they are the same with the Javaforge site lagging behind the Bitbucket site in releases. All bugs should be files on the Bitbucket site: https://bitbucket.org/mercurialeclipse/main
Originally MercurialEclipse was hosted on Bitbucket. Around the 1.5 release if I remember correctly Intland forked it to Javaforge. This is where the name HgEclipse originated. After some time the project reunited on Javaforge. Some time later just before the 2.0 release the project moved back to Bitbucket. There are a few reasons for moving back to bitbucket. One of them being that Intland did not do releases promptly and this caused confusion with users. Another reason is the JavaHg is hosted on Bitbucket and Bitbucket is otherwise a nice host.
Even though Intland has not directly contributed in a while I think they are still interested in hosting releases. Their update site includes Mercurial binaries whereas the Bitbucket one currently doesn't.