I alternate between using a bottom of the range pair of road specific shoes (Shimano RT32) and a touring/mtb shoe (Shimano MT41b) depending on the kind of ride I'm doing and, tbh, can't really say I notice any difference in stiffness at the level of riding I do.
Although the MT41b's have a little bit more flex in the sole which makes walking any distance more comfortable, this doesn't translate into any discomfort on the bike. I can do distances up to 80 miles without feeling any "hot spots" on my feet under the cleats. Getting the positioning of the cleats right is key in this - I spent time fine-tuning to get it right.
The only observation I would make is that I would choose a touring shoe with a grippy rather than a smooth sole, but this is just personal preference as I generally do a bit of light offroading when in touring mode, which invariably means getting off and pushing at some stage.