import bpy
import sys

argv = sys.argv
argv = argv[argv.index("--")+1:]

stl_in = argv[0]
fbx_out = argv[1]

bpy.ops.import_mesh.stl(filepath=stl_in)
bpy.ops.export_scene.obj(filepath=fbx_out)

bpy.ops.import_mesh.stl(filepath=stl_in)
bpy.ops.object.modifier_add(type='DECIMATE')

bpy.context.object.modifiers["Decimate"].ratio = number_of_faces_in_stl_obj/max_faces