This all depends on what you are trying to get out of your racquet.
As a general rule:
Polyesters:
Upsides: Durable. Lots of control. Lots of spin.
Downsides: Low powered. Harsher on the arm. Loses tension faster.
Multi-filament
Upsides: Very good playability. Wide range of price points. Softer on the arm. Very good tension maintenance. Closest thing to natural gut.
Downsides: Less control oriented. Certain multi-filaments can feel "mushy." Lacks durability for bigger hitters.
Synthetic Gut
Upsides: Economically priced.
Downsides: Average overall performance and playability. Not extraordinary in any category.
Natural Gut
Upsides: Optimum playability. Holds tension the best. Easy on the arm. Playability even at very high tensions.
Downsides: High priced.
"String Tension", measured in either pounds or kilograms, specifies the pressure by which the strings are secured to the racquet frame during stringing.
As a general rule:
Lower tension = more power, comfort and durability
Higher tension = more control and spin
Generally, the lower tensions range from 42 - 50lbs & higher tensions range from 56 - 64lbs