Which brake should I use, on a full suspension bike, if I broke my brake lever
If it's to get you home or to the bike shop, use the one that works and ride cautiously.  You may have to walk down some things. Trying to fiddle with it at the side of the road or trail is risky - you don't want to lose a screw. If you're considering long term use it's a bad idea to run only one brake in most situations, especially as you know the ones you've got are prone to failure.  

As you're taking about full suspension I assume you mainly use it off road. I'm not a mountain biking expert by any means, but on loose surfaces you don't have much stopping power from the back as a side is both easy and ineffective. On the other hand a light touch on the back brake is very useful, and overheating can be an issue on a single brake. On road you have the same effect to a lesser extent but the consequences of your second brake failing can be more severe (an uncontrolled meeting with fast-moving traffic).