"How do these engines compare?"
Stockfish will run rings around Fritz.
"What are their strengths and weaknesses?"
Stockfish is free, Fritz isn't. I'm guessing that Stockfish's evaluations would be a bit higher than Fritz's, based on what I've heard. I also think that Stockfish's endgame would be better than Fritz's.
"Which one is better for a beginner like myself?"
I don't know. What do you want to do with it?
Fritz 13 (1 core) vs Stockfish 5 (8 cores)
How do these engines compare?
What are their strengths and weaknesses?
Which one is better for a beginner like myself?