If you want to use our object model to show differences between two files, you'll want to look at the methods off of the Difference class in the Microsoft.TeamFoundation.VersionControl.Client namespace/assembly, specifically ones like DiffFiles, VisualDiffFiles, VisualDiffItems, etc.  Here's a simple example showing first a unified diff to stdout and then a visual diff that invokes in the appropriate registered third-party tool.

A sample run, showing a diff of 2 versions of the same file (2 different files would have been fine, too)

>difftwoversions jmanning-test $/60220-testing/test/foo.cs C11 $/60220-testing/test/foo.cs C12
Difference.DiffFiles - output to console
foo.cs
===================================================================
--- $/60220-testing/test/foo.cs;C11  (server)    3/7/2006 11:34 AM
+++ $/60220-testing/test/foo.cs;C12  (server)    3/7/2006 11:35 AM
@@ -1,6 +1,7 @@
 int foo(int bar)
 {
+    int j = 5;
     int i = 1;

-    return bar + i;
+    return bar + i + j;
 }
Press Enter to continue

Difference.VisualDiffItems - invoke registered third-party tool
[Note: the diff opened in in a separate window here]
Press Enter to continue

using System;
using Microsoft.TeamFoundation.Client;
using Microsoft.TeamFoundation.VersionControl.Client;
using Microsoft.TeamFoundation.VersionControl.Common;

namespace DiffTwoVersions
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length != 5)
            {
                Console.Error.WriteLine("Usage: DiffItems servername sourceserveritem sourceversion targetserveritem targetversion");
                Environment.Exit(1);
            }

            TeamFoundationServer tfs = TeamFoundationServerFactory.GetServer(args[0]);
            VersionControlServer vcs = (VersionControlServer)tfs.GetService(typeof(VersionControlServer));

            string sourcePath = args[1];
            VersionSpec sourceVersion = VersionSpec.ParseSingleSpec(args[2], tfs.AuthenticatedUserName);
            string targetPath = args[3];
            VersionSpec targetVersion = VersionSpec.ParseSingleSpec(args[4], tfs.AuthenticatedUserName);

            DiffOptions diffOptions = new DiffOptions();
            diffOptions.UseThirdPartyTool = false;
            diffOptions.OutputType = DiffOutputType.Unified;
            // Wherever we want to send our text-based diff output
            diffOptions.StreamWriter = new System.IO.StreamWriter(Console.OpenStandardOutput());

            Console.WriteLine("Difference.DiffFiles - output to console");
            Difference.DiffFiles(vcs, 
                                 Difference.CreateTargetDiffItem(vcs, sourcePath, sourceVersion, 0, sourceVersion), 
                                 Difference.CreateTargetDiffItem(vcs, targetPath, targetVersion, 0, targetVersion), 
                                 diffOptions, 
                                 VersionControlPath.GetFileName(targetPath), 
                                 true);
            Console.WriteLine("Press Enter to continue");
            Console.ReadLine();

            Console.WriteLine("Difference.VisualDiffItems - invoke registered third-party tool");
            Difference.VisualDiffItems(vcs, 
                                       Difference.CreateTargetDiffItem(vcs, sourcePath, sourceVersion, 0, sourceVersion), 
                                       Difference.CreateTargetDiffItem(vcs, targetPath, targetVersion, 0, targetVersion));
            Console.WriteLine("Press Enter to continue");
            Console.ReadLine();
        }
    }
}