In a large army i would squeeze out other things to take both since I believe in an all skink army they are both more than worth their points.
If you really had to choose one or the other, and that is essentially the question, I think it will somewhat depend on the rest of the army (post it if you like) and definitely depend on your personal preference and play style. The terradons are a good support unit, their drop rocks ability is kinda powerful against light opponents, they march block to control the movement phase better and hunt warmachines, lone mages and some other light support. The salamander is a great flanking unit that can cause panic very easily and has the potential to do a reasonable amount of damage.
Which do you think would support your army better? I would probably try both and see how they went. Everyone has different experiences with different units.