Eagle's Nest: Tour or No?

I have taken the tour with the Harpers so I cannot help with a comparison. I highly recommend the tour with David Harper. His wife Christine was our guide and it was one of the best tours we have taken in Europe. Having a keen interest in WWII was a factor for me I am sure but even those who just have a casual interest would enjoy this tour. I know this is not what you were looking for in your question but just wanted to let you know that this is a quality tour and well worth the money.

I, personally, would not waste my money on a tour. There is a bus from Salzburg Hbf to the Berchtesgaden Hbf and from there you can catch the bus to Kehlsteinhaus (Eagles' Nest). The first time I went up, at the bus landing area, there were English speaking guides offering to guide you for a few Euro. The guided trip was interesting, but when I came back a few years later with my wife, we didn't bother. About all they do is show you the room on the side, where Eva Braun entertained the wives while Hitler entertained the men in the large, octagonal room. She also explains that the marble fireplace in the large room was a gift from Moussolini, and had pieces chipped away by American soldiers with their bayonets.

The bus from Salzburg is called the Watzmann Express, and for €8,50, you can buy an RVO day ticket, which includes the RT from Salzburg to Berchtesgaden as well as the RT to where the buses leave for the Kehlsteinhaus. The RT to the Kehlsteinhaus via specially equiped mountain buses is an additional fare.

The tour with David and Christine Harper is by no means a waste of money. They are not the typical English guided tour that you pick up on the go. It is well organized, informative and iteresting. You have probably checked out the link below but it gives you a good idea of what you can expect as you take the tour.
http://www.eagles-nest-tours.com/EN/eagles-nest-tours/index.php?navid=2

If you plan on taking their tour and know in advance what day you will be there it would be a good idea to book in advance. They have a limited number of people who can ride on their mini bus. Some people had to be turned away when we were there for lack of reservations.

The Eagle's Nest tour may be better than some tours, but it is just a tour. You can do the tour yourself for a lot less money, and you can do it at your own pace, not dragged along by the group before your finish seeing things. Where the buses stop near the top I found plenty of guides offering an English language tour of the building for a few Euro. You don't have to spend €45 for a tour.

The tour starts in Berchtesgaden, not in Salzburg. The Watzmann Express ticket that gets you to Berchtesgaden also gets you to the base of Kehlstein at Obersalzberg. From Obersalzberg, the bus trip to the top costs €15.

I normally don't take a lot of tours preferring to do it on my own. Have taken the Scavi Tour at St. Peters, The Doge's Palace Secret Itineraries Tour in Venice, the Third Reich walking tour in Munich and several castle tours. Some of these were very good and some not so good. There are only two that I would do again if back in the respective areas and those two are David Harper's Eagles Nest Tour and the Night Watchman's tour in Rothenburg. To me and my wife they were worth the money. I don't offer opinions on tours that I have not personally taken.