... For improving the survivability of an animal companion. Damage resistance five adamantine, or plus two natural armor? Top of my head instinct one makes me want to say damage resistance at low level switching over to KC somewhere in the mid levels and switching back to damage resistance somewhere by the high levels.
DR is always going to be better at low levels, obviously, but nat armor only peaks above it at high level if you can get your AC high enough to keep enemies from hitting you at a 4-5, which is unlikely for an animal companion.
If you plan on barding them out in heavy barding and/or giving them some of your loot alottment for AC, the nat armor may become better sometime in double digits.
If enemies already need a 15 to hit you, pushing them to needing a 17 is a 33% drop in damage taken on average (for 15 damage per hit break-even with DR). But if they only need a 2, it's barely over a 10% drop (for a 50 damage break-even point!).
TL;DR: Grab the DR and leave it at that. Unless you optimize heavily for AC, then maybe nab the nat armor at 11th level or so instead.
EDIT: I should probably also say that at *very* high levels, if fighting lots of weapon-using foes, you may run into the issue of a +4 or higher weapon bypassing the DR. So at level 17+ the nat armor might be better even if you have mediocre AC simply because the DR is getting bypassed. This isn't a problem with a red dragon or pit fiend, but would be with a balor or any standard PC race of suitable CR.
Until you hit levels 12+ DR 5/Admantine is likely better. After that, damage is generally high enough that it doesn't matter as much anyway, and it can be relatively easily ignored via magic wepaons.
You'd be surprised, actually. The great and might Pit Fiend can't even bypass /magic without using his 1/year wish, though he does bypass /evil. Obviously such a creature should have access to a +1 amulet,but the standard stat block doesn't have one.
Even at high level it's probably about 50/50 whether they bypass your /addy DR or not, depending on campaign. Some campaigns will start to approach 100%, others might barely hit double digits. This makes the worthiness of the DR *very* campaign dependent.
TBH the Pit Fiend is kind of a wimp though.
The Balor, by comparison, will eat your DR for breakfast and come back for more.
And yet the Glabrezu, Vrock, Nalfeshnee, and Succubi he might surround himself with would not bypass. Heck, nearly every other kind of demon would not.
So, like I said, its usefulness depends on what you're fighting. If we assume 50/50 can bypass it at 20th level easing in from a hair over 0% at 10th level then DR becomes worse probably around 13th level or so.