I've had my Apex classic now for the last 8 years and it is rock solid. However, if I were in the market for a new Apex today, I'd invest in the Apex 2016 since it has some perks the classic apex doesn't out of the box. However, if you're on a budget and looking for a solid used controller at a low price, you can't go wrong with the Apex Classic/Gold.
My biggest gripe with my Apex Classic was the inability to do firmware upgrades wirelessly. I don't have access to an ethernet connection where my tank sits, so I depend on a wireless adapter for connection. When I have to do firmware upgrades, I have to connect my laptop to my Apex directly via ethernet cable and do the update, and while that is not complex, it's just another thing to do that I probably won't do until completely necessary. With the new Apex, you can just press a button and upgrade the OS.
The new Apex also has a built in conductivity/salinity port and probe included. This is something I wanted to add to my current set up, but a) it was pricey and b) I'm running out of room for more module boxes to hang inside my cabinet.
The new EB832 is also a cool feature I would like to incorporate for redundancy. Specifically the power management and power draw.
I'm a techie type of guy, so the more information collected that I can use to automate processes, the better. But like I said, if you need a solid controller at a lower price point and don't need the extra features and convenience, by all means, go with the Classic/Gold. I found that the Apex is a long term investment which pays itself off over the years. It's not a piece of hardware you upgrade every year. I remember paying $500 or something like that on my Apex Classic, thinking it was outrageous, but in the long term, it paid off by saving my tank of multiple catastrophes over the years. Heck, before I bought my Apex, I invested in the Reef Keeper Lite because of the low price, but then I ended up selling it all to buy an Apex Classic and lost money in the process for going for the lower cost solution. lol