An LLC is a legal entity offering liability protection with light administration; by default its profits pass through to the owners' personal tax returns. An S-corp is not an entity at all but a tax election that an LLC or corporation can make — profits still pass through, but owners who work in the business must take a reasonable salary, with remaining profit distributed without self-employment tax. A C-corp is a separate taxpayer: it pays corporate tax, and dividends are taxed again at the shareholder level, but it can issue multiple share classes and have unlimited owners of any type.
The choice usually comes down to what you want the business to become. Most small and lifestyle businesses do well as an LLC, possibly with an S-corp election once profit is high enough that the payroll-tax saving outweighs the extra administration. Venture investors, on the other hand, effectively require a Delaware C-corp — S-corps cannot have corporate or foreign shareholders or multiple share classes, which rules out standard funding structures. Converting later is possible but costs money and legal time.
A concrete example: your consultancy nets $140,000. As a plain LLC, roughly all of it faces self-employment tax. With an S-corp election you take a $85,000 salary and $55,000 in distributions, saving several thousand in payroll tax at the cost of running payroll and a separate return. Meanwhile the software product you intend to raise money for is formed as a Delaware C-corp from day one, because retrofitting it during a funding round is the expensive path.