Not sure how to compare your rating system (1-5+) but we do hit the big rocks out here in Colorado. No issues as long as you are:
lifted
armored
maybe bigger tires
We don't have to be locked but it does help in who feels like they can do a crazy route or just the hard route.
I used to cringe at the sound of my skids banging rocks, but I am used to it now. That is what they are there for and you know you are protected.
Can you mod the Rubi into a bigger beast and then do more crazy trails? Yes.
Can you mod the KJ into a bigger beast and then do more crazy trails? Yes and no. The KJ is limited in growth (bigger lift and tires) but it can perform great on those crazy trails (talk to USAFCOP).
We are riding trails with XJ's that have 35's and we do fine and sometimes they struggle more than us.
Remember, it is more the driver than the rig. The bigger the rig the more you think you can do but really can't.
How much is a loaded Rubi compared to a KJ? A lot more. Your full mods (armor, lift, locker in rear, steel bumpers front and back) to the KJ will still be cheaper than a Rubi.
Rubi (not the unlimited) on the Jeep website is $28590
KJ Sport on the Jeep website is $23685
You have almost $5000 in mods to the KJ to catch the baseline Rubi. You can do a lot of mods to the KJ. Possibly get the future 4" lift that we expect in 2007 from a couple of vendors that say they are going to work on it.
KJ = more comfortable on and off-road, less dust and dirt to breath in, expected to fail and a sense of pride when showing others that a KJ "can do."
Rubi = still bigger mod capable, the open air freedom, expected to perform well - but embarrassing

when you fail......and a KJ succeeds.

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New KJ Gear -
Desert Armor Flak2004 Renegade Light Khaki - 4" JBA Lift - Rock Lizard Products: Komodo Dragon Rear Bumper, Prototype Winch-mount Front Bumper, Super Skink Sliders - All J's Super Tranny Skid II - 31" Firestone Destination AT's on Moab wheels - Cobra 75W CB w/ stock Antenna mod
[b]Retired, US Military