What is the difference between a poolish and a biga?
I had to consult my copy of Bread Baker's Apprentice by Peter Reinhart to be sure, but here is a quote:


  There are two types of firm, or dry, pre-ferments and two types of wet
  pre-ferments. The firm pre-ferments are known as pâte fermentée and
  biga. The wet pre-ferments are called poolish and levain levure.
  
  ...
  
  Biga, an italian style of firm pre-ferment, differs from pâte
  fermentée in that it doesn't have any salt in it. Also, rather than
  cutting off a piece of finished bread dough to hold back as an
  improver, a biga is made specifically to be used as a pre-ferment.


So, it seems the main difference lies in the hydration of the dough, where a poolish is made with a ratio of equal water and flour.  There also seems to be a few other differences. A biga apparently uses .5 percent yeast to flour, while a poolish uses .25 percent.

Mr Reinhart does not say anything about a concrete difference in final result between these methods - just that a 'wet' pre-ferment is faster. Faster in the sense that in the same time-frame a poolish will develop more flavor and character than a dry pre-ferment.

Sadly I've personally only used a wet pre-ferment, since it is easier to handle. Just whisk it prior to bed time with a wire whisk and mix in the rest of the ingredients the next day. Having a full blown dough makes it more difficult to incorporate the rest of the ingredients.