It's my understanding that what's in our heart is what God is after, because that's where our treasure is: "For where your treasure is, there will your heart be also" (
Mat 6:21). After Paul was saved he realized that he still had the sin nature ("old man"). That's why he kept saying he didn't want to sin. Thus, what he wanted (not to sin) was the treasure in his heart. God knows we do not want to sin and that's where He wants us, not desiring to sin even though He knows we will; it's also the reason we do not see Scripture that relates believers as "sinners" (
1Pe 4:18).