Choosing a tip hardness also depends on what type of games you playing the most, and on what type of tables.
If you're new to the game, go with a medium to hard tip, till you figure out speed control and reading the table speed. Most new (male) players what to smash balls all around. I harder tip will last longer in that scenerio. On the other hand (female) players are a little more timid and a hard tip will last them a life time. (no pun intended)
If you've gotten the "pool bug" and have really started to focus on your stroke, you'll probably want to try a softer tip while applying "reading the table" techniques. While playing on 10+, 25+, 60+, 120+ different tables with soft tip, medium tip, or a hard tip, you'll begin to recognize certain simularities and dissimularities.
The more experienced players come prepared for any game, and any table type. How they're prepared is by usually having two shafts for their cue. One, will have a soft tip, the other, a harder tip. Because, certain games require finesse, other games will have extreme power shots greater than 50% of the time. Your break cue will probably have a more durable/harder tip, than either of the first two shafts.
All in all, tip preference comes with experience.