Deep Rybka vs Deep Fritz vs Houdini vs (other), which one to buy? I want to invest my hard-earned money to buy a chess engine which can help me to:


analyze games easily
experiment with different opening repertoire
strategical analysis
positional play


I don't expect an answer like, all are good enough to beat a human player, etc. I am not looking for an engine whom I can beat, as I know it's not humanly possible for a 1600 rated player to beat these engines, but my purposes are different (as given above).

So, if you have or use any of these engines, please let me know a comparative detail on which one I should go for.
I would suggest you look into some open source engines. And tools for analysis.  These will be free. Some will be quite fully featured and very very powerful: 

Stockfish. Rating: 3121
Critter (free) Rating: 3207

Also, look into SCID: an information database to which you can import millions of games and pair the database with analysis engines, add chess move-engines etc. 
    -https://en.wikipedia.org/wiki/Shane%27s_Chess_Information_Database

note 1: there are millions (literally) of master games which have been annotated, and can be downloaded in zipped .pgn files, then which can be imported to SCID for play through etc. So analysis, openings, endgames etc, can all be analysed via scid though whichever engines you decide on (no reason to only have one). 

note 2: Crafty finished in second place in the 2010 Fifth Annual ACCA Americas' Computer Chess Championships. Crafty lost only one game to the first place winner Thinker.

note 3: In various computer chess rankings Stockfish 2.1.1 was second or third behind the top gratis program Houdini and the free program Critter.

Good Luck!