Which is worse for credit score: Bank closure for inactivity, or consumer closure, of a credit card?

I think the answer could likely vary across scoring models, but in general, I would say neither is worse than the other, because most scoring models don't consider it. From the horse's mouth (dated January 26, 2017):

In the past, a statement that the account was closed by the lender was considered negative. That is no longer the case. People today open and close credit card accounts so often, that who closed the account is no longer a meaningful indicator of risk.

Once a credit line account is closed, as you pointed out it might affect your score, since your utilization percentage will likely go up. But the reason it was closed shouldn't matter.

Both are worse but it also depends on the scoring module.

When you close a credit card account or it gets deactivated, your credit limit for that card gets reduced from your overall credit limit which in turn increases your overall credit utilization ratio. If the overall ratio goes beyond 70%, your credit score might get impacted.

Canceling a credit card can cause your score to drop as it can lower the average age of accounts on your credit report, especially if it's an account that's been open for a long time.

This is because the age of your accounts is factored into your credit score, with longer payment histories bolstering your credit score. If you close an account that's been in good standing for many years, canceling it, and thus lowering the average age of accounts in your report, can ding your credit score.

It is highly doubtful that an account which is inactive would be left open by the bank for "years".

Most banks do a good job at winnowing out accounts with no activity in fairly short order. In many cases this can be as little as six months (with NO activity at all, mind you), while others may allow for longer, but almost certainly not years.

There might be exceptions, such as high net worth clients or clients who conduct other business with the bank, in which case the account might be left open as a courtesy.

It might be worth reading through the account terms and conditions to see if there is a clause on account dormancy.

As to your question about the effect on credit scoring, as long as the account was in good standing at time of closure then whether the bank or client closed it is a distinction without difference.

The more impactful aspect would be the age of the closed account relative to the rest of the accounts in the file. Anything that affects average age of accounts will likely have at least some small measure of effect (maybe barely noticeable) on the credit score. There are other considerations that determine this (number and age of remaining open accounts and so on), but since the mechanics of how the scoring models work (and this is highly dependent on which bureau and which model you're using) are proprietary and therefore kept secret, we can only make extrapolations based on what is observed.