#Hashtags are used when talking about something – a topic or keyword. Using a hashtag creates a link that leads to a page displaying an aggregated list of posts about that thing.
So if it is about the topic, then the hashtag is the way to go:
Microsoft releases #VisualStudio 2013 Update 2
On the other hand, @Mentions are used when talking to or about someone (the user account of a person, brand, group, etc.). The link created by a mention goes to that user account. The user is notified when they have received a mention in a post so that they can respond.
So if you are talking to someone directly, or mention a specific user in your tweet, you should use the @mention:
Thanks to @visualstudio for the map to Bill Gates’s house – I’m looting his safe right now!
The best tweets will use a combination of @mentions and #hashtags where appropriate to properly leverage the platform – that way readers can check out the people involved OR join the conversation about a topic.
Figure: This tweet utilizes @mentions, #hashtags, and a link. These will create higher engagement and let the reader further explore the topic