No direct experience with those three hotels, but from what I saw on the websites, the hotel in Hinterglimm is far and away the most appealing and seems to have the most facilities. The Trip Advisor reviews are almost overwhelmingly positive (and there are 67 "excellent" reviews, many of them from people who have posted many reviews, few of the suspicious one-hit-wonder reviewers).

One reviewer noted that the hotel picked them up at the Zell am See train station free of charge - you should explore that option as you wouldn't need to bother with the expensive or palaver of a car which you don't need anyway for any of your destinations (it might not be free for your short stay, but I suspect the charge would be reasonable and less than a car rental). If I were jetlagged, I'd prefer hanging out in the train, watching the scenery go by and having a drink over dealing with a rental car and driving on unfamiliar roads.

FWIW, we've been to Bad Gastein and of course opinions vary, but it didn't impress us at all. I would have say it was the low point of our last trip to the Salzburg region.

Thanks for posting the link to the Hotel Theresia - looks like someplace DH and I will have to consider

Finally, also FWIW, Fodorites have recommended the Hotel Friesacher outside Salzburg (near Schloss Hellbrunn). It's a spa hotel and might be worth considering - much easier to reach than Hinterglemm.
http://www.hotelfriesacher.com/hotel-anreise/?L=1

FYI, if you do opt to stay in or near Salzburg, treat yourself to an amazing dinner at Ikarus in Hangar 7. It features "guest chefs" from top restaurants around the world. Nothing else in Salzburg compares to it.
http://www.hangar-7.com/en/ikarus/

Thanks for the input and the Friesacher recommendation! It does look tempting also!

Hi, Where ever you go i feel sure that you will have a great holiday!!

Your idea of renting in Austria is a good one if you feel you need a car.

I can vouch for the two spas in the Gastein valley, both great but my preference is the one at Hofgastein. Also Tauern Spa Kaprun is worthy of a visit is close to Zell am See.
Austrian Trains will take you to the vicinity of all 3 spas.

If you are looking for well being also consider a visit to the Kitzsteinhorn glacier, the Krimml falls, and weather permitting Grosskglockner pass, to lift the spirits. A car would help to do these last two activities but not essential.

Let us kinow how you get on and have a great time.

Derek