In my opinion, based on what I've read, there is little difference between the S9 Autoset and the Airsense 10 autoset, They are the same machines wrapped in different skins. Same algorithms except for the For Her model which has an additional, less aggressive treatment mode "For Her". The Airsense series comes with a modem to call home which one can use to feed ResMed's My Air on line reporting to view a very basic sleep report online.
One advantage the modem equipped Airsense series might have over the S9 is the ability to do firmware updates "over the air".
The humidifier of the Airsense series is integral to the unit which some find a disadvantage in that it can't be removed to reduce the footprint.
Again in my opinion, a new Airsense will not treat your apnea any better than a new S9.
For starters, you should register here, get the SleepyHead software, and post some of your results to see if your current treatment is optimized. While your machine might be getting old, I think these machine are able to last far longer than 5 years under normal use. Personally, I would consider buying a new S9, if you could find one, over a new Airsense - same great machine, but cheaper. Others might disagree.