The threat model of seedsigner or coldcard are largely the same. The only real threat of any truly offline signing device is in the generation of a seedphrase and the device's internal random number generation or interepretation of dice rolls. You can e2e verify diceroll functionality of either device with third party software, or you can generate the seedphrase with plastic seedwords in a bowl, or separate software like sparrow. In any event, if you assume you can use the devices in an airgapped fashion and not rely on their integrity with taking common sense measures, what is the functional advantage of a coldcard over a seedsigner?