Select 2 computers, then run this report. It will show you which applications only exist on one of them.

Application Comparison.sql

-- https://gist.github.com/Colby-PDQ/c03e7eee22a73c9b1dcbe81aaa9b18e6

SELECT
	  Computers.Name AS "Computer Name"
	, Applications.Name AS "Application Name"
FROM
	(
		SELECT
			  Applications.ApplicationId
			, COUNT(Applications.Name) AS "ApplicationCount"
		FROM
			Applications
		INNER JOIN
			Computers USING (ComputerId)
		WHERE
			<ComputerFilter>
		GROUP BY
			Applications.Name
	) ApplicationCounts
INNER JOIN
	Applications USING (ApplicationId)
INNER JOIN
	Computers USING (ComputerId)
WHERE
		<ComputerFilter>
	AND
		ApplicationCounts.ApplicationCount = 1
ORDER BY
	Applications.Name COLLATE NOCASE

Application Comparison.xml

<?xml version="1.0" encoding="utf-8"?>
<AdminArsenal.Export Code="PDQInventory" Name="PDQ Inventory" Version="16.5.0.0" MinimumVersion="3.1">
  <Report>
    <ReportDefinition name="Definition">
      <Sql>-- https://gist.github.com/Colby-PDQ/c03e7eee22a73c9b1dcbe81aaa9b18e6

SELECT
	  Computers.Name AS "Computer Name"
	, Applications.Name AS "Application Name"
FROM
	(
		SELECT
			  Applications.ApplicationId
			, COUNT(Applications.Name) AS "ApplicationCount"
		FROM
			Applications
		INNER JOIN
			Computers USING (ComputerId)
		WHERE
			&lt;ComputerFilter&gt;
		GROUP BY
			Applications.Name
	) ApplicationCounts
INNER JOIN
	Applications USING (ApplicationId)
INNER JOIN
	Computers USING (ComputerId)
WHERE
		&lt;ComputerFilter&gt;
	AND
		ApplicationCounts.ApplicationCount = 1
ORDER BY
	Applications.Name COLLATE NOCASE</Sql>
      <ReportDefinitionTypeName>SqlReportDefinition</ReportDefinitionTypeName>
    </ReportDefinition>
    <Created>2018-04-26T16:31:07.0000000-06:00</Created>
    <Description>Select 2 computers, then run this report. It will show you which applications only exist on one of them.</Description>
    <IsNew value="false" />
    <Modified>2018-10-24T13:45:13.0000000-06:00</Modified>
    <Name>Application Comparison</Name>
    <Path>Reports\Application Comparison</Path>
    <ReportFolderId value="1" />
    <TypeName>SqlReport</TypeName>
    <ReportType>SqlReport</ReportType>
  </Report>
</AdminArsenal.Export>

Revision 2: Added COLLATE NOCASE to the ORDER BY clause to make it case insensitive.

Revision 3: Replaced instances of ON with USING.

Revision 4: Clarified the Description a bit.

Revision 5: Added a link to this gist.