The stepped up basis, as stated in the question, is _50_, not 10 -- that is, the stepped-up basis is the value of the stock or other asset _on the day of death_. As far as I can tell (I am not a lawyer or tax accountant), the stepped-up basis applies both to the trust and the beneficiary for capital gains purposes (assuming no estate taxes, etc., which all make things more complicated).
That is, it does not seem to matter for the capital gains calculation whether the trust sells the asset and transfers cash, or the asset is transferred: the basis is the value on day of trustor's death. It _may_ matter in terms of tax filings, because in one case the trust must file a tax return after the sale, in the other, the transfer out creates no taxable event per se.
However, remember that _income_ (e.g., dividends and interest) that the trust earns is separately taxable, again, either to beneficiary or to the trust itself. The trust may need to file its own income tax return depending how long it exists after the trustor's death.
Presumably, if the value of an asset _declines_ after the day of death, the beneficiary can take a loss if they receive it and then sell, or the trust could take a loss (?) by filing a separate return. This then involves a K-1 (getting over my head, here).
(There's also an option, maybe, to select 6 months after the death day as the day the asset basis is determined? -- IANAL, again, and that may apply only to some more complex trusts).
