I would advice you to take KSK (because it's my favorite train ЁЯША)
But here I will provide an unbiased comparison.... Members can help in adding more parameters...
Advantages of KSK over AP
1. Slightly less travel time
2....
more... Less halts
3. Easier to get ticket (compared to AP)
4. More nonstop action. You will witness skip of big stations like AGC,GWL
Advantages of AP over KSK
1. Legendary
2. Can catch from SC/HYB.... For KSK you need to go to KCG
3. More punctual
4. Rake is in better condition