FC's are best for calculation and logic that require no static memory or worth monitoring. Example: Finding the highest number or average of 3 numbers.
Value1
Value2
Value3
Temp1 = Value1 + Value2
Temp1 = Value3 + Temp1
Result = Temp1/3
Your not interested in the intermediate results etc.
This would be perfect for FC. I only do the simplest stuff in FC's. I use FB's probably 80% of the time.
If you need timers or other static data FB's are best. I do all my larger code sections in FB's. Multi-Instance FB's I use extensively. Very powerful and efficient.