I'm a huge fan of the Tunze ATOs. I've used the 3155 on larger systems (this would be good for your size) and I currently use the 3152 on my 14 gallon cube. They aren't the cheapest out there (although they aren't very expensive either), but I consider ATO one of the most critical pieces of my tanks so I'd rather stick with something that works well and is reliable than try to save a few bucks. At the end of the day, even a single failure (if no other failsafes are in place) can either wipe out your tank or wipe out your floor.
I'm actually much less complimentary of Tunze than most folks on here, since I've had issues with many pieces of equipment from them over the years. For ATOs though, they're my go to.