Between Stirling and Fuel Cell, we're basically weighing very quiet with totally quiet. The PN wanted fuel cell via the Type 214, but went ahead with China's Stirling stack as that was quite mature and proven.
Is there going to be a little more noise due to the moving parts of Stirling? Yes, sure, but the PN also weighed that the Hangors will operate in fairly busy waters, so there's enough masking to make that minor acoustic leak a non-issue in practical terms. There might be some other factors at play which we'll never know about either, e.g., possible use of lithium-ion batteries with the AIP.
However, we will likely see a Fuel Cell AIP solution come via the SWATS and next-gen SSKs, but the mission intent for those will be absolute silence.
