The Realpolitik of the situation is that if they expel Santos, he gets replaced by a Democrat immediately, who will swim to re-election. Gaetz is in a safe Republican district under a Republican governor, so they can replace him without causing any real problems.
Not to even remotely equate the situations, but it's the same reason why Dems went after Franken and are going after Menendez right now; it doesn't cost them anything to do so. They'd be much less likely to go after someone like Senator Manchin of WV, Senator Sherrod Brown of Ohio, and Senators Warnock and Ossoff of Georgia unless they were absolutely forced to, since doing so would mean the loss of the Senate.
And it is a genuinely messy question. How heinous an act is worth losing the ability to pass any legislation and make any judicial appointments? I really don't think there is a clear answer to that. A few Senators made the difference between Roe v. Wade standing and falling.