The OP wasn't comparing, just wondering if it is worth spending a little more to get more, but at what risk. What advantages.
It's a bigger jump in price to go to the 15K than the 18K from the 12K.
Personally, I think you have to decide if you need the additional output. If so, and if money is tight, the 18K is a viable option. It doesn't have the track record of Sol-Ark, but so far it's gotten positive reviews. Will, the site owner, loves it. If money isn't an issue, the 15K has more history behind it, and is basically the same as the 18K.
A major difference that both the 15K and 18K have is how they connect. Both have 200 amp passthrough. You can wire the grid right to the inverter. That allows you to use smart load, which all 3 have, a little differently, and separates your main panel from the grid.