Decide on impact or Crushing shredders
Impact shredders have the advantage of being less expensive but are a lot louder. These use a spinning blade similar to a food processor and you just feed the debris into the machine. Crushing shredders are quieter but more expensive. These models are self feeding and pull debris into the machine to crush it against a steel plate. Consider the type of material you are shredding when choosing as crushing shredders are better against heavier materials.
Gas or Electric
In this case electric shredders are the cheaper options but you will need to make sure the cord is long enough to move around the yard. Gas shredders have more power with the advantage of having no cord but they can require more maintenance and you will need to keep the tank filled with gas.
Check the Specifics
Pay attention to cleaning instructions and also check the limitations on the model you buy and sales associates can help you find what these are.