It is the same thing. Both your premiums and the hsa funds would be pre tax AND preFICA.
Agree with this.
Scenario 1: Your employer puts $1200 into your HSA upfront. During the course of the year, you pay $1200 less toward your HSA, and $1200 more toward insurance premiums.
Scenario 2: Your employer puts $0 into your account upfront. During the course of the year, you pay $1200 more toward your HSA, and $1200 less toward insurance premiums.
With most employers, your medical insurance deductions are pre-tax and treated the same as HSA contributions. If this is the case with your employer as well, both scenarios are a wash, tax-wise. If your medical insurance premiums are not pre-tax, it would be better from a tax standpoint to go with the unseeded plan and put more in the HSA yourself.
Assuming your medical premiums are pre-tax, I would probably take the $1200 up-front. If you decide to leave their employment part way through the year, you've gotten the full HSA contribution without "paying back" the full amount via higher insurance premiums.