For your booth at a trade show, the greatest long distance visibility that you can create is with a hanging sign.  So by all means, if you are allowed (usually you must have a island space) consider such a sign.  However, when you go out to design the sign, consider these key life cycle cost affecting variables:

1) Weight:  a)  If the weight of the sign is greater than 100 lbs, in most convention centers, you will be required to rent a chain hoist from the contractor that is required to do the rigging.  This chain hoist is used to move the sign from the floor to the air space high above the booth.

2)  Number of hanging points: b) If the sign hangs from more than one point, for instance a large rectangular sign might hang from each end, you will need one chain hoist for each point.

3)  Cost to rent each chain hoist is $500 -750.  This is only for the rental.  Costs of attaching chain hoist to the ceiling and moving sign into place are completely separate.

So limit the weight of your sign, by using tension fabric system or other lightweight structure and graphic substrates, and limit the number of hang points for your sign, so that you simplify the rigging and hardware required for hanging. 

Other hanging sign tips -

1) seriously consider rotating the sign.  Our brains are programmed to detect movement.  Movement will help your sign, company and booth be noticed.

2) consider lighting the sign.  The ceiling above the trade show floor is dark, and so is the air space.  For a few dollars more you can light the sign and keep it from disappearing in that dark space, and also help the sign stand out from other exhibitors.

3)  integrate the lighting into the sign design to further reduce running (life cycle) costs.  It is much less costly on a per show basis to own the sign lighting system.  Specify that lights are included with the sign, and that the lighting design is part of the initial sign concept.