A couple of things:
> FICA is 7.65%. Also, remember that this is only the case if you're under the social security wage base (about $127k). Above that you only save 2.9%, but it also then doesn't reduce your future social security benefit.
> The employer HSA contribution counts against the limit, so you'll "only" be able to contribute $6400 yourself.
> You probably don't pay income or FICA tax on your current insurance premiums, so I suppose to be completely correct you shouldn't consider tax savings on the entire HSA contribution, only amount of the contribution that exceeds the insurance premium savings.
> There is no 25% marginal tax bracket in 2018. If you were in the 25% bracket in 2017, chances are you'll be in the 22% bracket
All that together indicates you'll save (6400-2243.80)*(7.65%+22%) = $1232.31 off your taxes if you go with the HSA over the other plan.
You will need to use your employer's administrator if you want to payroll deduct, but after that you roll over to another provider. You can do as many trustee to trustee transfers as you want but there's often a hefty fee, or you can take the money out of one HSA and put it in another within 60 days, but this is limited to one such rollover every rolling one year period (starting the date you do the rollover, not a calendar year).