I am glad you are having fun while learning some APO technical things. It is always an advantage for functional person to know technical things.
Both /TSINIT and /TS_PAREA_INITIALIZE opens the same screen which is used for initializing the planning area. Difference is /SAPAPO/TSINIT is a Transaction Code where as /SAPAPO/TS_PAREA_INITIALIZE is ABAP Program which can be run using Transaction Code : SE38.
Happy Learning 
Thank you
Satish Waghmare