[dice]

Without tests for toxic substances, it would be impossible to
say what might be in land fill compost that you would not want
in your garden. (They probably make it out of collected yard
waste, plus maybe manures from commercial livestock farms
of one kind or another; out here there would be a lot of
composted commercial wood waste in it, too, bark and
sawdust, but there is probably not a lot of that produced
in Iowa.)

Horse manure is usually safe. You might ask whoever provides
it whether they lime their stalls (some race tracks do that to
keep odors down). If they do, that makes it an alkaline soil
amendment that can raise pH in the garden.

Unwanted possible toxic components aside, both are excellent
soil amendments, with the horse manure usually having more
nitrogen and phosphorus and the compost more potassium.