Hi dvtruc,
in my experience there is little differance in performance or effeciency, the only draw back with single Halls screws is the increased maintenance requirement compared to a single screw of the same duty. This is offset a little by the loe cost of the service spares. If you have a number of Halls screw requiring the same tool kit costs are reduced, if not it is very expensive to buy the tools just to maintain one or two screw compressors.
Stick to a twin screw such as a Howden unless you are able to do the overalls yourself and you have the tools.
Regards. Andy