Dave - generally speaking, a 7-blade reel is best suited for cutting bermuda, tiff, bent grass and zoysia. The 5-blade reel is best suited for cutting wet grass or areas that have a great amount of moisture (rain). The prevailing rule: the tougher the grass and the finer the cut you want, the more you want the 7-blade; the wetter the grass, the taller the grass, the chunkier the lawn, the more you want the 5-blade.
Wet Climate: 5-blade
Common Bermuda: 7-blade
Hybrid Bermuda: 7-blade
Zoysia Grass: 7-blade
Bent Grass: 7-blade
Junky Grass: 5-blade