Agilent publishes a lot info on line about different sources. One can by found here at
http://www.agilent.com/cs/library/technicaloverviews/Public/5991-2106EN.pdfThe extractor source uses a extractor lens with applied voltage instead of a drawout plate held at ground potential.
The extractor source will give you close to a three fold sensitivity boost. However, if you use a target tune such as stune.u or custom tune to achieve target rations, the extractor source sensitivity is about the same as a inert source.
if you are not doing trace level work, it is overkill in my opinion.