Another thing to note is that there will be a completely new app model in SharePoint 2013. So when you're not looking on deploying next week, you might want to take a look at that. SharePoint 2013 also has better out of the box targeting for mobile devices. There have been no announcements yet about converting current SharePoint Online environments to SharePoint 2013, but I expect that to happen somewhere in 2013.
About the sandboxed solutions, I don't expect Microsoft to say: we're not supporting them any longer so when we upgrade SharePoint Online they'll break. The upgrade for 2013 is a database attached upgrade (thus: new farm). I think for SharePoint Online that means they'll upgrade environments gradually, probably leaving it up to you when you want to upgrade (as with BPOS). My development 2013 environment still has sandboxed solution support and Microsoft has taken some effort to provide 2010 backwards compatibility, so I don't think they'll skip it alltogether. But still; for your question I'd definitely recommend checking out the app model.