Pretty similar offers from both companies. Both are offering options rather than RSUs.
Both going through huge growth right now. But, I feel like The Trade Desk might be better for professional growth.
That said I disagree with @xFGv12 about upside... Public or private is ultimately irrelevant to value of options. All that matters for that is growth potential. The HRMS industry is ~$8B/yr and is highly saturated with mostly the same kind of companies (is Gusto truly a fundamental game changer compered ADP or Workday?). Even if Gusto is wildly successful and overcomes the many heavyweight incumbents, they have a relatively low ceiling on their growth potential, unless you believe the category itself will dramatically scale, or Gusto will pivot to another business with a larger TAM.
Global advertising on the other hand is a $750B industry ($1T in <10yrs) and is ~3% tapped for The Trade Desk's way of doing business (i.e. "programmatic"). Growth potential is orders of magnitude higher for The Trade Desk. There is a very rational case for the entire industry to be transacted this way eventually. We're still in the top of first inning...
If you chose Gusto and it doesn't turn out to be everything you hoped, come on back. We'll leave the light on for you :)