Considering this is an opinion question, I am happy to give my own. In my opinion, Milton's poem "On the Morning of Christ's Nativity" is closer to the Metaphysical style of poetry. In fact, it is very complex. Let's take a look at some definitions in order to get us started. First, the definition of Metaphysical poetry:
highly intellectualized poetry marked by bold and ingenious conceits, incongruous imagery, complexity and subtlety of thought, frequent use of paradox, and often by deliberate harshness or rigidity of expression
This could certainly be said about "On the Morning of Christ's Nativity," a definite Metaphysical poem. Yes, it is youthful and idealistic, but definitely prone to conceit (an unusual comparison)! This idea of a conceit always helps to define a metaphysical poem! Milton fares away from the traditional nativity idea of mother and child, but takes a bird's-eye view and gets more cosmic about the whole ordeal. Further, Milton realizes that Christ was going to have to die and be raised from the dead to bring Christ's mission to fulfillment. Let's take a look at part of the poem itself:
See how from far upon the Eastern road
The star-led wizards haste with odors sweet:
O run, prevent them with thy humble ode,
And lay it lowly at his blessed feet;
Have thou the honor first, thy Lord to greet,
And join thy voice unto the angel choir,
From out his secret altar touched with hallowed fire.
How very interesting it is that it is the MUSE (a very Greek poetic term) who Milton asks to inspire the "star-led wizards," otherwise known as the three wise men who carry the gifts for the Christ child. It is this combination of both Christian lore and Greek mythology that gives Milton's poem it's more cosmic and metaphysical style, breaking it apart from the simplicity that accompanies poets such as Jonson.