earthdistance or PostGIS for find * within point and radius

I have a database with the earthdistance contrib module installed and
I need to find records whose long & latitude are within a given
distance of a zip code. From the documentation on earthdistance, I
believe it is certainly possible to do a "find points within a
distance of another point" using the ll_to_earth, earth_box, and
earth_distance functions. What would be the advantages to installing
and using PostGIS instead?

I would like to avoid adding another dependency to the system and I
think earthdistance will be able to do what I need. However, I'll
certainly try PostGIS if it has a significant performance advantage.
Can anyone talk a bit about the pros and cons of earthdistance and
PostGIS?

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John L Cheng