Overleaf has its advantages (e.g. better git integration) but I haven't heard much praise for their editor. In my own experience, the Overleaf editor is slower and more cumbersome to use than ShareLaTeX. It's much harder to rearrange files and compile time to PDF is much slower. You also can't set your main doc to be in a subdirectory of the project.

I hope this partnership means we'll soon have the ShareLaTeX frontend on top of the Overleaf backend. That would be a truly powerful combination.