Categorized by theme is more beneficial. To argue otherwise is absolutely idiotic. Unfortunately, many chess players are lacking logic away from the board. Many people will come here and try to argue otherwise. Don't listen. They will try to tell you that no body is going to tell you, in an actual game, in a tournament or competition, that there is mate in 2. This is STUPID!
You can't simulate competition. The point in solving puzzles is to drill the patterns into your cognitive thinking process. You should react to forks, pins, skewers, etc. the same way that you would to traffic lights and signs.
It's just a fact that people learn by repetition. One needs to see the same pattern or theme again and again and again before reaching the point that they will spot it just at a glance. If you have the opportunity to fork your opponent's king and rook, it should just leap out at you the same way arithmetic does.
It's also important that the puzzles are well organized and that you understand how they are oganized as you are progressing. You should see a pattern within a group of puzzles.
Ooh, I surely don't want to be an idiot, so I guess I must agree with you, right? 
What is more beneficial?