What a great question!! i am so glad that a session will normally include both.
The bare handed over the knee spanking is erotic because you feel your Mistress's skin and humiliating because you are being spanked like a little boy.
The flogger, paddle and whip. on the other hand, makes me feel that i am proving to my Mistress how much pain i can take for Her.
i really do not see one as better than the other - just different. And of course, my favorite thing is . . . whatever pleases Mistress the most.