Angels can physically perform what the Holy Spirit can't.
Beg your pardon? How's that?
The Holy Spirit lives within a person. An angel does not. Angels exist outside human bodies in the natural and supernatural realm.
Angels are God's messengers. They deliver God's communication in a direct, corporal way. For instance: the Angel Gabriel appeared and gave news to a stunned Mary. He said something like, "Fear not, you'll be the virgin mother of Jesus." Shocking news, right?
Gabriel later appeared to Joseph, Mary's intended, in a dream, and after the birth of Jesus, a body of angels filled the skies one night and announced the arrival of the Messiah to shepherds.
So, they can talk to us. Another role of angels is that of guardian.
People have reported seeing angels use their protective skills, as in the case of Elisha.
"And Elisha prayed, 'Open his eyes, Lord that he may see,' They the Lord opened the servants eyes and he looked ans saw the hill full of horses and chariots of fire all around Elisha." NIV. 2 Kings 6:17.
Angels can work physical, mighty things that the Holy Spirit can't, but they aren't around all the time to give communication or protection. The Holy Spirit is!
Upon an acceptance of Jesus, we begin a relationship with Him. He sends His Spirit to dwell within our soul. The Holy Spirit brings us evidence of our connection to Jesus.
The Holy Spirit takes up residence and never leaves us. His purpose is to lead, teach, comfort, and convict us of sin. He also helps us pray. He knows God's language and speaks to Him in words we don't know. He interprets our thoughts and needs to the Father.
Not so with the Holy Spirit.
I may not always have an angel nearby, after all, Sarah has others to help besides me, but I'll always have His Spirit with me.