general EI question

I'm going to order a 46GT and the CH Auto Advance EI was mentioned a few times but what is the difference between the CH unit and Roy Sholl SS unit?
I couldn't find anything on the Brillelli website but I/m new to gassers and the diff may be common knowledge to the gasser guys but not me
Thanks
Dave P
P.S. finally got tired of cleaning castor snot off my planes at the end of the day and decided not to needlessly grease up the EF YAK 54 - 74