---------->Short, Quick and To the Point on SC<---------- *Edit* Well, it seemed short enough in the fast reply box...
Sensory Chamber
<>Cloak: Same deal as Silence above. You may get that initial surprise, but honestly, you aren't meant to be the king of sneaking.
--I will admit, you can pick off a lot of LA marines by cloak+devour+flee/recloak+gore/rinse/repeat, but this novelty of a strategy wears off real quick when your initial shock factor wanes and the comm starts sending out groups of marines with weaponry, scans, and mt. You COULD pull this off with 1 hive, but truthfully, if your team is doing 1 hive SC, you really shouldn't be going Onos.

<>Scent of Fear: Good way to avoid throwing yourself into a collection of 6 marines. Helps you to pick your battles.
-- Probably the best/safest upgrade an Onos can have from the SC.
-- My personal choice if I'm not sure 3 hives are going to be available/maintainable. Makes it so you're not going to essentially suicide rush and keeps you safe when you're pounding on buildings.

<>Focus: Sheer obscene power.
-- Now the Onos gore does 180 damage to each marine. I'm not sure, but I believe that only two hits are necessary to kill an Armor3 JP/LA marine(confirmation please).
-- Further stacked on this, if I recall correctly, each building takes a solid 360 damage per gore (confirmation on double structure damage would be nice). Nothing to be said of crippling the comm's Obs and AA in a few short seconds and then fleeing the scene.
-3hive Required- The real kicker to being a Focus Onos is a nifty little combo involving charge and gore. Charge does an obscene amount of damage and combined with a Focus gore, you can take down a squadron of LA/JP(if grounded or flying slowly) marines in <5 seconds. Furthermore, if you have Adrenaline, you can float through a marine base, destroying at will and keep going, as Focus reduces the overall use of energy in destroying a building.

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Update to Regen/Carapace: Personal preference is the key here, with inherent regeneration. These examples basically assume equipped marines, but not such that every marine has an hmg, shotgun, high upgrades, jetpack/heavy armor, or a bio-disintegration ray.

<> Personally, I go with Regen.
--I may not be able to stay in battle more than say 4-6 seconds at a time, but over a minute, I will have been able to fight at full capacity probably 4-5 seperate times (sans gorge/other) rounding out to about 15-30 seconds of fighting a minute.
--With a gorge that knows what they're doing (slot2, +point-towards-onos, +attack, +stays-atleast-a-full-onos-length-away-so-the-onos-has-room-to-fight [too bad some of those are not actual commands...]), you can stay fighting pretty much indefinitely, and what's better, is this constant battle could be maintained by the hive, a single gorge, or 1-2 DC.

<> Carapace can keep you alive maybe another 1-3 seconds, but you have a much longer downtime without a gorge/healing station/hive to stitch you up. Over a minute, you're probably reduced to around 8-10 seconds of actual fighting.
--With a gorge/healing station/hive, you might be able to pull off fairly constant battle over a minute, but with an hmg or a couple shotguns, you won't be able to without more healing sources.