Power vs. Control
Choosing the correct tension of your strings requires you to be familiar with your level of play and consistency. In general, the tighter the strings, the more control and LESS power.
Tight Strings = More Control
If you are a consistent player with medium to fast strokes, you may consider stringing your racket at the tighter end of the manufacturers suggested range as you probably do not lack power, but may need more control. If you have slower, broader strokes and need help generating power, then the looser string tensions are probably more appropriate.
Loose Strings = More Power
Strings tend to loosen pretty quickly with play. The more resilient or “playable” strings will loosen quicker than the strings designed for durability.
Tighter tensions will also reduce the size of the “sweet spot” and, thus, cause the racket to be less forgiving.