First of all, Fritz 13 is a GUI + engine, Rybka is an engine only. So you can't really compare them.
I suggest that you try to find the best GUI for your needs. All good GUIs allow you to use any engine and there are a lot of free ones. Stockfish, Houdini 1.5, Critter, and IvanHoe are all free engines, on par with Rybka, and stronger than Fritz.
Silentiarius is correct... you're comparing apples and oranges.
I'm going to assume, then, that you just wish to compare the engines themselves.
The Fritz engines (Fritz 11, Fritz 12) usually are not quite as strong as today's other top engines, namely Houdini, Stockfish, Critter, and yes, Rybka.
For that reason, I would select Rybka over any Fritz engine, including Fritz 13. (But I'd suggest Houdini, Stockfish and Critter over Rybka.)
Here's a link that you've probably seen, comparing the strength of more than 200 chess engines, many of which are free:
http://computerchess.org.uk/ccrl/4040/
But yes, you will also need a GUI to run your engine. There are many to choose from, and many of these are free. WinBoard, Arena (ug), Tarrash, Scid vs. PC, and Kvetka are just a few freeware GUIs to choose from.
And yes, you can also choose the Fritz 13 GUI, or the Shredder GUI, or Aquarium, or several other commercial GUI.
I probably have a dozen different GUIs on my PC... I'm constantly evaluating them all the time.
I myself prefer Houdini with WinBoard for most of my engine/GUI needs.
I guess it all comes down to what you want to do, what features you need, what your computer experience level is, etc. The Fritz 13 GUI might be overkill for you, for example.
I've read some amazing reviews and info regarding both Fritz13 and Rybka4. I'm sure there are many players in here of both software...which would you recommend, and why? Thanks again for all the help and support i've gotten on this forum, it is much appreciated!