Super strong engines aren't for playing against or studying games they play against eachother. So yes, it would be a waste of money :p
Rybka 4 is much stronger than Fritz 12 -- although like rdecredico said, they're both so strong that it doesn't make much difference to players like you and me.
The only reason to go for Fritz, if you've already got Rybka, is if you prefer the GUI (Fritz has both GUI and engine). I personally like the Fritz GUI, I tried Aquarium and hated it. The Fritz12 engine is not quite as strong as Rybka4 but is comparable. I very much doubt you will learning much by watching the two slug it out, much better to spend the time on master games, annotated ones if you can find them
I have Rybka 4 Extreem, and I enjoy playing against it seeing how long I can last, also playing against other strong chess engines, so would it be a waste of money to also buy the Deep Fritz 12, does anyone know if it is stronger then the Rybka I have? It seems to be capable of doing alot more other things then Rybka. Thanks for any input!
One other thought, is any thing to learn studying games between two strong chess engines, then master's play? I must admit for me it is captivating with often exciting climax.