I use both, depending on the guitar and the preference of the person commissioning it. Same price--there is no significant difference in time and material cost. Fitting the dovetail requires a bit more sophisticated ability to visualize in three dimensions, and more experience to get it up to speed. But I regularly use a dovetail joint on my least expensive guitars because I like making them.
Bolt on is easier to take apart, but what is the big deal about an extra 20 minutes after 15 years?
Dovetail is lighter weight, allows for a more elegant heel shape, and pleases traditionalists.
Neither comes apart on its own when well executed. I use this stuff to keep the bolt from getting loose (do not use the red stuff):
