I bought the gold and I think the salinity probe is worth it. Yes your ATO will keep salinity constant but changes will indicate problems. I could tell when my ATO was out of water, for example, when I went on my two and a half week roadtrip. Don't count on it to be 100% perfect. All the probes are more for trends and changes in those trends.
I haven't set up everything I can do on the Apex yet but being able to control everything remotely is cool and I added a camera so that's another level of security that I appreciate. Can't wait to start adding alarms and fine-tuning my lights and pumps.
Join the Neptune Systems forum and read the "New APEX Users: Avoid these common mistakes!" post.
I'm going to assume you have an older system based on your "used" statement. So, yes, I agree, hard wire the ethernet.