Should I use Dropbox or Aperture to back up iPhone photos?
If you have your Photostream set up in Aperture and on your iPhone, the photos will also get automatically downloaded to your computer. Dropbox has a similar feature, so if you wanted automatic backups, you could set up both Photostream and Dropbox sync and then back up your Aperture library (from Photostream) to your external HD. That way you have three copies, one of which you cannot modify from your phone.