Chickens are more popular. Quail are more economical.
As far as sales, you would probably do better with chickens. They just add a bit of country to anywhere. Most people prefer chickens over quail, eggs and meat. I'm not talking just poultry enthusiasts, I'm talking the general population.
Quail usually require a license and more special pens. More expensive feed, but if you crunch the numbers, they are more ecomical for eggs and meat. It's just the eggs are small and not practical for today's cooking.
Chickens are less economical to keep, but are more popular and don't require a license in most cases. When it comes to selling to the general public, chickens are the way to go, people will want to buy eggs and sometimes birds for cooking. Chicks in spring and laying hens in summer.
Don't get into the hobby purely to make money, it's not worth it unless you enjoy it.
-Kim