Warp on Shephard, Heavy is the correct choice. 5m AoE blast is large enough that there is no reason why you should take unstable, when their CDs are the same. +2m Warp Explosion blast just isn't enticing enough to sacrifice +25% damage.
On teammates though, I'd take the smaller cooldown on Warp. 25% reduction in CD is effectively a 33% increase in DPS, so at that point that 160 damage unstable warp does more DPS than Heavy. Note that even with Warp Explosion, most enemies don't die in one shot at full health in insanity with Heavy Warp.
Reave on anyone though, Area all the way. It's powerful enough (almost as strong as Heavy Warp against minions) that it will strip any minion's armor within one shot even in insanity, which makes it immensely useful when a horde of melee starts rushing you. Fact that it CCs when targeted against organic health is just gravy; main purpose is to AoE and help keep you alive so that you can keep firing your gun. With instant travel time, I can fire it out of cover even at 1/3 health and still live to keep firing my gun until the next CD, which makes it a godsend to significantly lower the time you spend fighting 'trash' battles.
And really now, Area Reave will outdps ANY single target skill assuming you can get it to hit more than 1 target. And with its instant travel time and 3m blast, it ain't that hard. As for cover, fact that you have AoE means you actually have some cover penetration; not like you need it anyways, because in insanity, vast majority of the time the enemy is out of cover trying to shoot your butt to oblivion.
As for single target bosses, I've come to conclusion that Reave actually SUCKS against them. This is because they seem to enjoy further duration resistances than normal ON TOP of Insanity duration resistance, thus offering you lower damage on single targets that matter.
So really, there is no point in taking Heavy Reave. It's supposed to be good against single tragets, but heavy warp already does a comparable if not better job already. Probably the only upside to it is that it offers you 90 extra health per single target than Area Reave, but considering its single target, many times you won't even have a target thats down to health to actually take advantage of it. With Area, you can Reave a target on health and maybe catch his friend's armor, setting up another target of reave on next CD.
As for Reave vs Warp, then it depends on class. And considering there's only two classes with Warp, I'll talk about them in detail. Note that its not really a question of Reave vs Warp, as it is a question of Reave vs another skill.
For Adept, I am of the opinion that there is no other skill better than Area Reave. Fact is, most enemies this time around are organics (unlike in ME1 where majority were synthetics), and around half the synthetics use armor anyway. Main antagonist targets are organic as well, so vast majority of the time, you can put Area Reave to good use.
And when used correctly, you can have it keep you alive even at low health and even under fire, significantly increasing your overall dps because you don't have to hide behind cover for long periods of time. Which is a problem with Heavy Warp Ammo, as that extra damage is no good when you're sitting behind cover for more than half the time just because you can't take a hit. And unlike stuff like barrier, it allows you to go on the offensive with each power use and Reave still keeps you alive better, since you can recharge your health by 200+ points twice for every 1 barrier you can cast.
(Probably the only time Barrier and such is better is against single bosses, but the devs seem to give you more places for cover than usual during these times that I felt at no time I thought Barrier would've made a difference in that particular situation.)
You still want Heavy Warp though, because its still superior against sub-bosses and higher, warp explosion is your main weapon against synthetic health, and the fact that there really is nothing better to spend points on.
For Sentinel, it depends. Artillery Sentinels (ie. Guardians) have much more use for Area Reave, but NOT for the same reasons as the Adept. Reave is useful to Guardians because it gives them a much a needed Area CC against organics and AoE armor/barrier-killer, something that Sentinels sorely lack to be good artillery. Coupled with Area Overload which is CC against synthetics and AoE shield-killer, they can pretty much strip anyone's defenses and then proceed to make them useless once they hit health... all at 3m radius blasts! Also frees them up for a fifth skill, which usually is Heavy Throw to instakill some problematic enemies in ledges, a quick stun for others.
Assault Sentinels (ie. Raiders) have more to gain by using Warp Ammo for offense, as they let their guns do their talking. They need their global CD ready for Tech Armor anyways. since they're much more reliant on that for their survival due to their more in-your-face tactics.
tl;dr
Heavy Warp > Unstable Warp for Shepard. Opposite for teammates
Area Reave > Heavy Reave period
Area Reave > any bonus skill for Adept/Artillery Sentinels
Assault Sentinels should just stick to Heavy Warp