the main difference between both techniques is the tar production. Updraft gasifier produce higher amount of tar that generate problems in the clean up gas systems 

There are books and references on this subject. The answer by Dr. Andres Fullana is along right lines. But you can use updraft systems for thermal applications with hot gas. 

The advantage updraft gassifier are:

good thermal efficiency, small pressure drop, and little tendency towards slag formation and

the downdraft gassifier has own advantage such as:

flexible adaptation of  gas production to load and low sensitivity to charcoal dust and tar content of fuel

Regards,

Maizirwan

IIUM malaysia