Do you see any distinction between mandatory disclosure of compensation for endorsements, and mandatory disclosure of interest when recommending a stock?

Yes, because the latter has a direct nexus with a commercial transaction.

I was under the impression that, if paid for, political speech (which is the type of speech the First Amendment was intended to protect) already came with disclosure requirements that had been contested and ruled constitutional.

The appropriate analogy isn't a choice between being able to say "Hillary Is Great" or nothing at all, it's a choice between having to say "paid for by Super PAC for Issue" if accepting their cash to broadcast their view on Hillary, or not accepting the cash and saying whatever you like about Hillary.