I started with a Biocube. It was a great little tank that I bought used. I ran it for 4-5 years before upgrading and then used it as a QT tank before selling it. There were some great accessories made for them like media racks if you're willing to spend a bit vs using egg crate. In my case, I used an LED retrofit kit and was able to keep and grow any corals including a couple acro's if I put them right at the top.
Highly suggest an ATO. Stability is key and on nano's it can be hard to manage. Plus, 2 gallons can last a couple of weeks depending on where you live lol!
Good luck!
Looking into an ato as well. I noticed a drop in my tank already it's been going 2 days. I top off with freshwater?