The S-Corp in particular has some attractive features: it simplifies equity grants to employees compared to an LLC, and taxes are easier to deal with in a C-Corp (there's also a sort of notorious salary-vs.-distribution trick people place with S-Corps to reduce their taxable income).
The LLC is incredibly easy to set up; you can probably get one via 1-click on Amazon now.
In the only company I founded that took serious VC, I didn't handle any of the legal, but the sense I got was that legal for a real VC round is so innately expensive that the S-to-C conversion isn't a big deal by comparison. It's most convenient for everyone if you're not even incorporated, but that's their problem (it is dumb to do business without incorporating); if you're worth funding, nobody is not going to fund you because of the cost of converting to their preferred structure.