Part One of Three: A frame challenge:
Settling star systems in the Orion Arm of the Galaxy is a bit like settling the aurora borealis.
When you look at a spiral galaxy from the outside, the spiral arms look very impressive, being much brighter than the spaces between them. Thus at first sight it is logical to assume that the stars are concentrated in the spiral arms and their are few stars between them.
Thus in James Blish's "Okie" or Cities in Flight stories first written during the 1950s, it is said that Earthmen have spread out along Arm II of the galaxy.
But in a later Blish novel from the 1960s a character notes that the spiral arms are mostly visual features of a galaxy and that the star distribution is more uniform throughout the galactic disc.
In most science fiction stories of interstellar colonization, humans colonize naturally habitable planets of other stars. And science fiction writers who want high scores in the Sliding Scale of Science Fiction Hardness:
https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness
Will have their human colonists only colonize planets of stars which are capable of having human habitable planets. Since it took Earth billions of years to develop an atmosphere breathable by humans, planets that are naturally habitable for humans should only orbit stars which are billions of years old.
And only some spectral classes of stars can last for billions of years.
Thus naturally human habitable planets should orbit spectral class G and some class K stars, and maybe class F stars on the shorter lived side and maybe class K and M stars on the dimmer though longer lived side.
And all stars in our galaxy orbit around the center of mass of the Milky Way Galaxy. Stars which are closer to the center take less time to complete one orbit, while stars that are farther from the center take more time to complete one orbit.
So if a straight line is drawn between two stars and the center of the galaxy at one time, millions of years later the star closer to the center of the galaxy will have pulled ahead of the farther star, and so the line will no longer be straight.
Spiral arms also orbit the center of the galaxy. They are regions of higher density interstellar gas and dust, where new stars form. New stars are formed in a wide range of mass and luminosity, including short lived highly luminous stars. So spiral arms contain many might young stars, and bright clusters of young stars, and nebulae that reflect light from young stars, and so they are brighter than the spaces between spiral arms.
Stars generally form in open star clusters. And the older a cluster is, the higher the percentage of its stars which have escaped from the cluster due to gravitational interactions with passing stars. So the star clusters which stars form in gradually dissipate and their stars become spread out over a vast volume of space and orbit the center of the galaxy independently instead of as a group.
Long before any planet of a star can become habitable for humans, that star will have escaped from its birth cluster and orbit the center of the galaxy independently.
The young stars which are part of spiral arms are mixed up with many times their number of older stars that formed millions or billions of years earlier. So the density of stars per cubic volume of space is almost exactly the same within a spiral arm and outside it.
The galactic disc of the Milky Way galaxy is more or less arbitrarily a hundred thousand light years in diameter. The stellar density decreases gradually with increasing distance from the center of the galaxy. And the stellar density also decreases, but much more rapidly, with increasing distance "above" or "below" the central plane of the galactic disc. Thus the galactic disc is often arbitrarily said to be one thousand light years thick and one hundred thousand light years in diameter.
So if humans spread out from Earth settling on naturally habitable planets, they will settle on planets of disc stars, and won't care whether those disc stars are within spiral arms. The region of human settlement will be a spherical area which gradually increases in diameter, until the "top" and "bottom of that sphere reach the ill defined "upper" and "lower" edges of the galactic disc. Then the zone of human colonization will become a flat cylinder bounded on "top" and "bottom" by the "upper" and "lower" limits of the galactic disc, and expanding radially throughout the galactic disc.
Part Two: One Possible Reason to Settle Spiral Arm Stars.
But maybe human colonists won't colonize naturally habitable planets. Maybe some advanced civilization terraformed many planets of young stars in spiral arms to be habitable billions of years before they would naturally become habitable. Maybe all the naturally habitable planets have their own life forms which have evolved there, and only the terraformed planets are considered to be open for colonization.
In that case humans would colonize only young star systems which had been terraformed by the advanced civilization, and so they would spread out colonizing terraformed planets of spiral arm stars instead of planets of the general disc stars which were already taken.
Part Three: A Second Possible Reason to Settle Spiral Arm Stars.
Or maybe humans don't colonize naturally habitable planets or planets terraformed by some advanced alien society, but settle in stars systems that have many small objects suitable for space mining and build artificial space habitats to live in.
The younger a star system is, the higher the concentration of heavy elements will be in that system. The younger a star system is, the less likely it will be to have planets already formed, and the more likely it will be to have billions of small rocky planetesimals which would be much easier to mine than planets.
Thus for a society which builds artificial space habitats very young star systems may be the most desirable. And so such a society might only colonize young spiral arm stars, and spread out along a spiral arm, instead of colonizing disc stars in general, and spreading out in a spherical and later disc shaped volume of space.