My understanding is that an S corporation is the easiest for a new firm, and can always be changed to a C corporation later. What are the reasons not to do it this way?
The firm under consideration is in profesional services (primarily consulting) starting with a sole owner, eventually growing organically out of profits.
Also, are there tax reasons to pay out the income as dividends rather than salary?
Incorporation Legal Tax S Corp
There are definitely tax reasons to pay out dividends and not salary (primarily - lower tax rates on qualified dividends, which may expire at the end of the year, and the absence of the FICA and payroll taxes).
However, personal services C-corporations are taxed at higher (flat) 35% rates, which may not be the best rate with lower income corporations. Officers are required to be paid salary in corporations.
As to S-Corp, while you can avoid portion of payroll/FICA taxes, you're required to pay yourself reasonable salary, which, according to recent court decisions, should definitely be at the market level. In a recent decision, an accountant was required to pay himself salary of up to the social security maximum level, essentially rendering (almost) worthless the S-corp savings.
Having said that, unless the law prohibits, you should consider LLC/PLLC. While (taxation-wise) similar to S-Corp, it has the benefit of not requiring paying salaries to self, and less costs associated with maintaining the entity. LLC can be converted to a corporation either for tax purposes or as an entity pretty easily.
Do consult with a lawyer and a tax adviser on all issues.