Umm actually due to the relative proximity of hull-mounted sonar to the blade and shaft, they're more affected by sound interference. Towed always offers the better resolutions.towed sonar must be deployed 1st and can be used on certain speed if you dont want the blade interfering the towed sonar capabilities. So thats why both Ficantieri and ALRI are aiming for the FREMM ASW deal.
Indeed focused on one platform should be the ideal solution. But i think MoD is trying to get many hull asap, although collecting various type of warships like a showroom.