> Is letting the pretax grow untouched better than the benefit I get from using pretax dollars for health costs?

Yes.

> So essentially if I never claim ANY reimbursements through HSA or use my HSA for anything, would the cost difference between pretax and after tax be significant enough to make using the HSA funds worth it?

I don't exactly know what this means. There's no such thing as an after tax HSA. And you claim reimbursements later in life.

> How can I best determine the value of each decision?

If scenario A: contribute to HSA. Invest account balance. Pay medical expenses from other cash. Save receipts for medical expenses along way. Reimburse yourself later.

If scenario B: contribute to HSA. Pay medical bills from HSA as you incur then.

Scenario A wins.