When you perform an ACH, you are actually sending money to the central bank, waiting for the funds to clear the overnight process, then the funds are sent to the destination bank. This process takes 2-3 days but ensures the sending bank has enough liquidity to cover these debt transfers. If the sending bank fails, you are SOL until the central bank steps in with FDIC/SIPC (usually immediate, but we saw 2008)

With a wire transfer, the sending bank earmarks funds and moves it to a special account. Once the central bank confirms, the destination bank takes money out of their account (this is where the delay occurs, as sometimes the bank must setup a repo with the central bank) and sends it to the recipient. If the sending bank where to fail, the recipient still has their money. Now its up to the sending, central, and destination banks to duke it out for the money.

I am referring to US wires in this case