Doton takes an additional second to execute over Raiton due to needing to enter three mudra commands as opposed to two. So even in a worst case scenario, you're losing 1 GCD every 2-3 times you use Doton over Raiton. That additional 60 potency is less than the 210 potency you'd get from just a Spinning Edge.

Also it's worth mentioning that in order to even get that extra 20 potency, the boss also has to stay in the floor circle for the entire cast, and this isn't always going to be the case. Bosses jump around, they go untargetable, and tanks sometimes just have to move them, meaning that if you lost even 3 seconds of the boss standing in the circle, you've lost potency over using Raiton. And neither of these things even consider what was already stated that by using doton, you also don't get your stacks of raiju ready.

Basically put, never ever ever use doton when there's only one target.