FTM 2012 introduced a feature branded TreeSync. Basically, whatever you have at Ancestry.com (assuming this is what you mean when you write "online") will also be what you have in FTM 2012. In other words, it's all the same tree, just different ways of accessing it (FTM or Ancestry.com).
If you're a half-way serious family historian or simply interested in backing up the work that you've put into your tree at Ancestry.com, then I would recommend purchasing 2012. It will download copies of everything from your tree at Ancestry.com (including all of your citation media like census record images). FTM 2012 is a super-enhancement for Ancestry.com.