Difference between uc3843 ( duty cycle =100%) and UC3844 (duty cycle =50%)

Hello.

     I feel confused   about the  different duty cycle between UC3843 and UC3844 .     Can you  tell me  where  I  must use the  50% duty cycle (UC3844) ,and where I must use the  100% duty cycle (UC3843) ?

I want to use UC3843  in my  new product . Will you please tell me is it suitable for my  require?  Some detail information as follows:

1.  Vin = 18 V DC to  36 V  DC

2. Vout1 = 15V  , 100mA.

3. Vout2 = 5V, 2000mA

4. efficiency  =70%