Personally, I vote for the bodies staying. Until such a time as they are revealed to be energy projections of some eldritch being or some such, them dissolving strains my suspension of disbelief a little too far. =P

Plus, I usually look at it as an artistic liberty, rather than outright fact. We don't see them dissolve often, and we see their heads mounted, as mentioned above, so I usually explain it away as Monty and Co. not wanting to take the time to include mountains of corpses in a relatively short episode. And the trailer, after all, was a really early working draft, so it shouldn't be used to judge a lot anyway.