I have to agree with Zogger, either go real big, or stay small. And the biggest reason, the permits, license, insurance, etc will eat you up. And there really isnt a lot of difference for all these insurance and license costs between a big operation and a medium operation. Trust me, I know.
Six years ago, I was right there in the middle. Hired a guy who did nothing but split and pile while I hustled the tree jobs and dropped the wood off at the lot. Made good money removing trees, but after running the numbers for two years I realized that I wasn't hardly paying for my time keeping the books. I cut way back on production after investigating into some more serious equipment like a processor and skid steer.
I keep it small now, split and deliver less than 50 cords a year, take only cash, and I keep quiet about my operation with the tax man. I do okay, make some pretty good pocket money, and gives me a way to get rid of the debris from tree jobs. But there is little profit to be made if you buy your wood in log form or off the stump. One thing goes wrong and you got a spendy repair to make, and you may find yourself paying for the privilege of working.
To be honest with you, I dont know if I would even fool with firewood except for the cash money and its a great way to pick up tree jobs. Every delivery to a new customer gets a fridge magnet and I have had great success in picking up jobs that way.