In this case the highest tech on offer is AIP, "Air Independent Propulsion" tech that Germany is a leader in, which uses hydrogen fuel cells to allow for the sub to remain submerged for weeks at a time. The benefits are huge, unlike a typical non-nuke sub you get some of the benefits of a nuke plant, with the benefits of a much smaller, cheaper, easier-to-run vessel. The AIP is also extremely quiet, more so than even a well shielded nuke plant, so this style has real advantages for hunter-killer subs.