No one has given a shot yet, I will shoot from the lip...
Stronger, the biscuit. It is a big ole hunk of rubber.
Safer, the spool. When the rubber deteriorates and breaks the engine is still captured by the mount itself. From my limited experience you may not even realize the spool mount is "broken" without a real close inspection.
If I had a choice of two broken biscuit or two broken spool mounts, the spool wins, hands down.
C