TheBloodGod wrote:
 You're leaving out a few important pieces.
 Yup. Out of laziness I admit, because we're discussing the second best choices only ;-)  
 One: Vindicator gun is stronger, that is why it's only 24" Range. The Defiler's 72" Range leaves it a lot safer from many close-range anti-tank such as troopers or rhinos with meltaguns, etc.
 
 ...which is the basic Chaos problem I mentioned. You cannot give the vindicators proper cover or reach target saturation by spamming other dangerous vehicles - like cheap razorbacks or sane dreadnoughts. The real beauty of obliterators is that they are infantry in an infantry-centric army.  
 It does have AV12 instead of AV13, but that's because a Defiler is meant as somewhat of a sniper, where a Vindicator is a kamikaze close-range thing.
 Two: If the vindicator gets its gun blown off It's basically gun-less.  If the defiler gets its gun blown off it can still have a base of 5 dreadnought close combat weapon attacks.
 
 Right. But if you use the defiler for sniping, it has exactly 0 close combat attacks, because it is not in assault range. That's why I am a big fan of dedicated units. So if I were to use a defiler (won't happen, I got too many heavy support choices already), I'd go for the one completely tooled out for close combat, so I would not be tempted to stay away from the opponent and waste my expensive 
cc ability.  
Three: Defiler's gun isn't AP2, but it IS S8 which means it will insta-kill to 99% of the 2+ save things people wield. Since being on a bike or having a mark of Nurgle (the two common ways for a guy having T5 and a 2+ save) both still suffer insta-death at S8, there is not really a difference between the AP2 of a vindicator and the AP3 of a Defiler.
 I am not sure which unit you refer to with T4(5 or 6) and Sv2+ - a Chaos Lord in terminator armour with 
MoN on bike? Anyway, insta-killing does not remove the armour save. So, with AP3, you go from 0% chance to save on a sv3 unit to 83% to save on a  sv2 unit. 
 S8 AP3 is great against: Plague Marines, Nob bikers, Wraithlords, those few 
SM chars without eternal warrior in power armour. The first is a pretty good use, the second is semi-good, because wraithlords these day get their 4+ coversave (elfzilla -> wraithguard screen), the third has a 3+/4+ cover save (exhaust cloud) and the last one... who uses those? You might also find that all of these are footslogging units. Slow. 
 The difference between AP2 and AP3 is 
TEQ. That means Shrike's fleeting termis, 
CC godfexes, obliterators... three very common enemy units.
 The difference between S10 and S8 is not the insta-kill, not the better to-wound (although it is nice wounding wraithlords and carnifexes on 2+ instead of 4+/3+) but the fact that you can properly destroy vehicles (meaning: land raiders, obelisks - two units that got their 5th edition renaissance). 
 All in all, I stand with my statement: If you really decide not to use obliterators, the vindicator is the weapon of choice, because it is a dedicated unit filling a clear-cut role, one that is supposed to be filled by heavy support: Killing the heavies. A defiler with battle cannon is not dedicated enough. A 
cc defiler is, but has the same problem: Target saturation. So they are even. Yet a defiler does survive an assault on the heavies because they get to hit back. Shooting from distance works, though. 
 I hope my line of thought can be understood, it's really late, I should be asleep by now ;-)