Question 1:Why did God spare David and rejected Saul even though they committed almost the same offence.

This is a question and answer segment. It will spans for as long as there are interesting questions to answer. 

Answers: 

When David ran away from Saul he went to Ahimelech the priest. He told him to give him 5 loaves of bread or  anything for food because he and his men were hungry. Ahimelech told him he has nothing except the holy bread which should be eaten only by the priest or descendants of Aaron. But David and his men took the bread for food. Reference_1sam21_4

There were also times David sacrificed unto the Lord and they weren't consequences for his actions. And one may wonder why. 

Saul on the other hand was told to destroy the Amalekites. He was to wipe them off the face of the earth because of their evil deeds against God's own people(Israelites) He disobeyed God by keeping the king the best animals. God sent Samuel to rebuke him by telling him obedience is better than sacrifice and to hearken is better than the fat of ram. Reference_1sam 15

The second instance Saul disobeyed was when he was to go and fight the Philistines after killing their leader. Samuel told him to wait for him at Gilgal, although he waited for seven days but Samuel did not arrive at the speculated time. He was pressured due to the departure of his soldiers. He decided to make the burnt offering sacrifice Samuel ought to do. Samuel was displeased and told him that he should have waited for him and because of what he did he  has lost favour in the sight of God. Reference_ 1sam. 13

Looking at the two character, one would think David's offence is grievous because it has to with the runnings of the temple but Romans 14_23b says “anything done without faith is sin.”

Saul made the sacrifice because a bulk of his soldiers left him and he began to feel agitated. He sacrificed out of lack of faith and disobeyed Samuel's instructions. God wanted a man who would obey him in all Circumstances. “Fear God and not men because God will be the judge of the world”

David on the other hand did not eat the show bread out of disdain or disregard to the lord.  Nor did he scarifice to the lord out of disobedience. He did everything out of faith believing that he won't be punished. 

Romans 1_27b, Habakkuk 2_4 says “the just shall live by their faith.

 Jeremiah 17:10

“But I am the Lord,

and I can look into a person’s heart.

I can test a person’s mind and decide what each one should have.

I can give each person the right payment for what they do.”

The heart of a man and his intent is most important to God. 

“Guard your heart above all else, for it determines the course of your life.”

It's very important that you be careful about what you think,

because what you think controls [MET] the things that you do.(T4T)

Keep your heart safe and guard it with all diligence,

for out of it is the wellspring of life.(unlocked literal Bible) Proverbs 4_23

“it's not about our good intentions it's about God's instructions”_ Oluwabiyi