They are both as fictional as each other. Most of the old Testament is made up stories about the past after the Jews established themselves in Judea and were freed from Babylonian captivity by Cyrus the Great.
For what it is worth, the Old Testament has possibly the greatest writing in history. Much more quotable than the New Testament. Also the old Testament has some hardcore stuff like
Psalm 137 wrote:By the rivers of Babylon we sat and wept
when we remembered Zion.
2 There on the poplars
we hung our harps,
3 for there our captors asked us for songs,
our tormentors demanded songs of joy;
they said, “Sing us one of the songs of Zion!”
4 How can we sing the songs of the Lord
while in a foreign land?
5 If I forget you, Jerusalem,
may my right hand forget its skill.
6 May my tongue cling to the roof of my mouth
if I do not remember you,
if I do not consider Jerusalem
my highest joy.
7 Remember, Lord, what the Edomites did
on the day Jerusalem fell.
“Tear it down,” they cried,
“tear it down to its foundations!”
8 Daughter Babylon, doomed to destruction,
happy is the one who repays you
according to what you have done to us.
9 Happy is the one who seizes your infants
and dashes them against the rocks.
The Catholic Church actually doesn't say the full thing at mass anymore due to the Vatican 2 reforms.