I choose both. Here's why. The Kingdom of Heaven always existed. It is eternal. David's Throne was God's throne according to the OT. But the Messiah had not yet been resurrected to sit on His Throne in Heaven until NC times. So think of it as Christ taking David's Throne in Heaven during his resurrection. Where he remains until the end of the world, where he then remains seated on his own throne forever in the New Heavens and earth. Think big..........
That's not both, that is saying it always existed which then you must have misspoke when you said it began at Pentecost.
All dying in faith enter heaven at time of death.
Now here is the problem with this thinking:
"And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven." (John 3)
Prior to the resurrection, all dying in faith went to Paradise. After the resurrection, Jesus led "captivity captive" and took the OT saints into heaven. "Thou hast ascended on high, thou hast led captivity captive: thou hast received gifts for men; yea, for the rebellious also, that the LORD God might dwell among them." (Psalm 68:18) Take special note of the last statement "that the LORD might dwell among them" and compare to......
"For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:" (Heb 8) Of course this prophecy is ALL New Covenant and was fulfilled through the death, burial, resurrection and glorification of Jesus Christ and began only after those events had been fulfilled. Christ was never "In" the believer until the New Covenant, He couldn't be until sin was washed by the blood and removed. God cannot dwell with sin. Only then was a person fit for Christ to enter into them as one and the person was fit to enter into heaven.