I am no expert as I have only been in the business (manager) for a few years now, but I have attended both the SSA and ISS expos. Both were held in Las Vegas at different locations.
In my opinion the ISS is the only way to go. They are much better organized and I learned a lot more from the ISS Expo.
I will not be attending a SSA Expo again, unless they make a LOT of drastic changes to their organization (and offer me free attendance and hotel to make up the the last one!!
)
Just my .02