To me Signlab seems easier to operate sans training than AI. It may not have all the capability of AI as I don't get into super complicated muli-layered graphics, but to me the bottom line is that SL and Flexi are sign making programs. AI, CDR, Photoshop, etc. have lots of cool features but are not specifically designed for large scale graphics.
I also find that the artwork my customers bring me from those programs always seem to have issues, whereas stuff I export from SL seems to always work with no problems