Ordinarily a startup should be a C corp. It’s cheaper to be an LLC, but if you plan to succeed, you may as well do things right from the start.
With an LLC profits don’t get taxed twice like in a regular corporation. So it makes sense to be an LLC if you expect to have substantial profits, but don’t expect to grant options, sell shares, or get bought. Consulting firms and law partnerships are often LLCs. YC is an LLC.