What's the Difference Between a Guardian and a Saviour?

Anyone who performs an act of rescue or deliverance may be described as a “saviour” e.g., the judges Othniel and Ehud (Judges 3:9, 5) and King Jeroboam II (2 Kings 13:5). God Himself is called “Saviour” (Isaiah 43:11; 45:21–22; 60:16). Scripture also identifies Jesus Christ as our Saviour:

For unto you is born this day in the city of David a Saviour, who is Christ the Lord. (Luke 2:11).

...waiting for our blessed hope, the appearing of the glory of our great God and Saviour Jesus Christ, who gave himself for us to redeem us from all lawlessness and to purify for himself a people for his own possession who are zealous for good works (Titus 2:13–14).

We need a Saviour because God is too pure to abide sinfulness: “Your eyes are too pure to look on evil; you cannot tolerate wrongdoing” (Habakkuk 1:13). Without a Saviour, God’s word to us would only be “away from me, you evildoers!” (Matthew 7:23).

There is no one righteous, not even one (Romans 3:10; cf. Psalm 14:1).

If we say we have no sin, we deceive ourselves, and the truth is not in us (1 John 1:8).

Since God is holy, He cannot tolerate the presence of sin, and that is why Jesus came to Earth as the sinless and perfect Son of God.

God made him [Jesus] who had no sin to be sin for us, so that in him we might become the righteousness of God (2 Corinthians 5:21).

He himself bore our sins in his body on the tree, so that we might die to sins and live for righteousness; by his wounds you have been healed (1 Peter 2:24).

For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive by the Spirit (1 Peter 3:18).

Angels are capable of sinful rebellion against their Creator and that is why no created angel could possibly atone for the sins of the world. Only God can do that:

In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made. The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth (John 1:1–3, 14).

The uncreated and eternal Word of God, who is of the same essence as God, took on human flesh and came to the Earth in human form – Immanuel, which means God with us.

“For in Christ all the fullness of the Deity lives in bodily form (Colossians 2:9).

Angels are created beings and they perform various functions. Most angels in the Bible are portrayed as messengers, although Michael the Archangel is described as contending, fighting, or standing against evil spirits and principalities (Daniel 10:13; 21; Jude 1:9; Revelation 12:7). Angels praise God (Psalm 148:1-2; Isaiah 6:3). They worship God (Hebrews 1:6; Revelation 5:8-13). They rejoice in what God does (Job 38:6-7). They serve God (Psalm 103:20; Revelation 22:9). They appear before God (Job 1:6; 2:1). They are instruments of God's judgments (Revelation 7:1; 8:2). They bring answers to prayer (Acts 12:5-10). They aid in winning people to Christ (Acts 8:26; 10:3). They observe Christian order, work, and suffering (1 Corinthians 4:9; 11:10; Ephesians 3:10; 1 Peter 1:12). They encourage in times of danger (Acts 27:23-24). They care for the righteous at the time of death (Luke 16:22). Source: https://www.gotquestions.org/angels-Bible.html

But no created angel is ever identified as THE Son of God. Indeed, the Bible clearly shows that the created angels worship Jesus because the pre-mortal Jesus was never created:

Hebrews 1:5-8 draws a clear distinction between Jesus and the angels: “For to which of the angels did God ever say, ‘You are my Son; today I have become your Father’? Or again, ‘I will be His Father, and He will be my Son’? And again, when God brings His firstborn into the world, He says, ‘Let all God’s angels worship Him.’ In speaking of the angels He says, ‘He makes his angels winds, his servants’ flames of fire.’ But about the Son He says, ‘Your throne, O God, will last forever and ever, and righteousness will be the sceptre of your kingdom.’” The hierarchy of heavenly beings is made clear in this passage—angels worship Jesus who, as God, is alone worthy of worship. No angel is ever worshipped in Scripture; therefore, Jesus (worthy of worship) cannot be Michael or any other angel (not worthy of worship). The angels are called sons of God (Genesis 6:2-4; Job 1:6; 2:1; 38:7), but Jesus is THE Son of God (Hebrews 1:8; Matthew 4:3-6). https://www.gotquestions.org/Jesus-Michael-Archangel.html

Far from being able to save humans, Hebrews 1:14 says angels are all ministering spirits sent to SERVE those who will inherit salvation. Yes, angels are able to rescue or deliver God’s people from harm, but they cannot do anything to save sinful humanity. If Jesus was only a created angel or merely an imperfect and sinful man, then his death wouldn’t even atone for his own sin, much less the sins of another. But Jesus is no mere man; he is God in human flesh. As a perfectly sinless man, Jesus alone was able to atone for the sins of mankind.

In mainstream Christianity, only Jesus is the Saviour, whom God the Father sent to the world to become flesh and blood to save humanity by dying on the cross. By necessity, the Saviour has to be full human flesh and blood. This Jesus is also the Son of God, which is another requirement for the Saviour, but no angel is the Son of God.

In contrast, God assigns a different purpose to guardian angels, which is to care the people who will inherit salvation (Heb 1:14) after the Saviour saved them. The contrast is well taught in Hebrew 1.

Another distinction is from the service they perform for believers: Saviour saves us from the power of sin and the power of death (see Rom 6:1-11 and Heb 2:14-15), a service that angels cannot perform since angels never become human. Once saved, believers are still under attack from the dark angels although God somewhat restrain them, put them "on a leash" in a manner of speaking. Guardian angels are one of the means that God uses to protect our earthly life from the dark angels. Therefore the services are completely different.