The Jetboil Sol was discontinued in 2015. For our current backpacking stove recommendations, go here.
The Jetboil Sol is an iteration of the Jetboil Flash. Overall we prefer the Flash because it is a better value, has more capacity, can often be found on sale, and is cheaper to replace parts down the road.
There are three main differences between the Flash and the Sol. 1) The Sol uses a smaller 0.8 liter cup as compared the 1.0 liter cup on the Flash. 2) The Sol uses a new "Thermo Regulate" burner that gives "consistent heat down to 20 degrees F." According to Jetboil, it only boils water a little faster: the Sol boils 16 oz of water in 2:15 while the Flash takes 2:30. 3) The Sol weighs 3.5 ounces less than the Flash. Consider that most of this weight savings comes from a 20 percent smaller cup - not a big improvement.
Considering the Sol costs $20 more than the Flash yet is smaller, we don't think its the best buy unless you think you will notice saving 3.5 ounces. The better system to buy if you don't mind dropping cash to shave weight is the Jetboil Sol Ti ($149, 8.5 oz, 0.8 L). This system is 40 percent lighter than the Flash, 50 percent more expensive, and has 20 percent less capacity. The Sol is less versatile, slower, and not as storm-proof as the MSR Windburner.