When depositing to an HSA, is there a difference in the taxes paid if you do pre-tax direct deposit via employer versus a post-tax (deductible) deposit on your own?

I have cash on hand that I want to put in my HSA. I'm just curious whether it is better to fill up the HSA via direct deposit through my employer, which is pre-tax, or if I should just use cash on hand because it is deductible (even if I don't itemize). If there is no difference tax-wise, then I assume I should just deposit the cash.