Look at the example below.
Which tile should you discard?
45 pin is ryanmen, therefore you should choose between 12 wan and 12 pin. On the surface both are penchan, discarding either group shouldn't matter. Let's first count the number of effective tiles.
a) discard 12 man: drawing 36 pin to tenpai
b) discard 12 pin: drawing 3 wan, 3, 6 pin to tenpai
Therefore, discarding 12 pin is the correct answer here. This has 50% more effective tiles. Both are penchan, but discarding 12 pin is better, the reason is because 1245 pin is a bad shape with effective tiles overlapping.
As 12, 45 taatsu both require 3 pin, once you discard 12 pin, the number of effective tiles will not decrease. In fact, overlapping of effective tiles is an important efficiency theory in Japanese mahjong, you should pay attention to this during actual play.
Beginners often hope to draw 5 wan for a 3 sided wait and discard 23 sou, this is incorrect. In fact, because 3467 wan has effective tiles overlapping, you should discard manzu instead.