I'm in the same situation. I still have a 350D (yes they still exist), and plan to ugrade in the near future. In the race are the 6D, and the upcoming APS-C models (70D? 7D MkII?). I only shoot as an amateur, and I'm really asking myself if it is worth to upgrade to a FF (6D).
DOF is one point, but I think that the DOF is already quite shallow on a APS-C with a decent lens; especially when you are close to the subject. There is a range in distance where the FF would be helpful, but is it worth the money?
Reach; this is only a question of money. Are you willing to pay twice as much for a lens, to get the same reach with a FF, as you would have to with a APS-C? Croping is not an option; why should I pay for Sensor-area I do not use?
IQ/Noise. This is the biggest issue. But living with 8.5MP for 7 years showed me, that the quality of an Image is not measured in ISO, Noise or dynamic range. Sure, Images from a FF look really nice. I compared the pictures from FF to APS-C on The-Digital-Picture.com, and the the FF looks much better (no surprise), but then I looked at the comparison tool at dpreview and there the verdict was not so clear anymore. Sure FF is always better, but by how much?
If you are a pro, then it is easy to deceide, but as an amateur I'm still asking myself if it is worth to upgrade.