While and LS is certainly not a rice engine, it is a sports car engine. It just seems like a strange thing to think about a corvette slowly crawling over obstacles, especially at elevated rpm's. Granted, it wouldn't be crazy rpm's, but still, not quite the easy going idling.
There is no doubt a hemi or ls would be faster, and be make the Jeep more like a sports car, that's not really what I feel is lacking.
Maybe my mentality is a reflection of my age. FWIW, I don't ride crotch rocket motorcycles at all. Instead, I enjoy riding my carburated Heritage Softail Classic.
Over 90% of our swaps use the Truck/SUV LS based engine, mostly the 5.3. The 5.3 engine has over 300 hp and 300 ft lbs of torque. Most 5.3's have AFM which turns the engine into a 4 cylinder cutting the displacement in half. In a light JK AFM works well, we use hydraulic engine mounts to isolate the annoying 4 cyl vibrations. In a heavy JK AFM/MDS does not do much for mpg. In general 5.3's will get similar or better mpg than a 3.8 due to the efficiencies of a larger displacement engine under load.
The Truck/SUV engines are very docile, they idle at 500 rpm and produce most of their torque below 3,000 rpm. These engines are designed to haul millions of half ton GM work trucks on regular gas for hundreds of thousands of miles with minimum maintenance and repairs. The all aluminum 5.3 with a 6l80 will add about 50 lbs to a JK. The 6l80 is a marvel of a transmission and a separate discussion; however, it is an important variable in deciding if you want a swap.
We recently have installed a few LS3 "Corvette" engines. I'm with you, why do you need a high revving 500 hp monster in a 3 ton SUV? It makes no sense but man is it fun to drive. The 480 version HP LS3 we just completed has a rough idle and would not be ideal for crawling, but above 1,000 rpm it is dead smooth and by 2,000 rpm it pulls hard, and keeps pulling to near 7,000rpm. It really needs to be experienced to be appreciated. The sound and torque of a V8 and the feeling of being pushed back in your seat, it's like no V6. It's not for everyone but it is a thrill. Surprisingly this same engine can get good mpg, 16+; the Vette guys are topping 30 mpg with this engine.
A V8 simply transforms the JK into a different class of vehicle IMO. I agree with Jim, the 3.8 will get your JK to where it needs to go. Many are happy with the performance of the V6 and with power adders it can be improved. We just did gears in a 4 cylinder YJ and when I took it on the freeway I realized how much better the 3.8 in the JK is than some of the older Jeeps. Jeeps were intended to go off road, not cruise at 80 mph loaded with a ton of gear and people, and Jeep built them for that purpose.
But in the end I could not be happy with a 3.8 in my JK. I bought a new 09' Rubicon JKUR and hit the road on a family vacation. Loaded up we left Vegas for NY. By the time we hit northern UT the trip turned sour. At 8,000 ft outside Park city I got pulled over for going too slow, I was nearly floored and my wife was nagging me to go faster. I was afraid I would throw a rod the way the 3.8 was revving and I couldn't go faster, this was with stock 32" tires and 4:10's. The cop let me go and the rest of the way to NY was miserable. Fighting the winds in WY, getting passed by a VW van in the 80 mph zones in TX, I had enough. I loved the vehicle and with a baby coming I needed a 4 door since my trusty 2 door would no longer suffice. I tried to like the engine, I bought a programmer, talked to many engine performance companies and drove Hemi and SC JK's. We service Hemi and SC JK's so I am familiar with them. The 3.8 is not a bad engine, it was put in a vehicle that smothers it. In a 2 door manual JK the 3.8 is good, in a 4 door JKUR auto it needs help.
The Hemi is a good option and has got much better with time. I personally like the lightweight, compact size and aftermarket support of the LS. I'm a GM guy at heart and grew up with Camaro's and Vette's. I like the 5.7 VVT Hemi, it feels the best of the Hemi's I have driven. The early 6.4's did not feel right too me, but I think the most current ones have got the bugs out. Our swap also has evolved with electronics that are allowing the LS to be installed almost plug and play. One advantage the LS has is it's stand alone engine performance capability in the event of a Chrysler can bus failure, the Hemi on the other hand is easier to integrate.
I could not be happier with my 09" Rubicon now. With a 6.2 LS we drove the same trip to NY. Going through the mountains in UT, what mountains? The 6.2 just goes, cruises at 80 mph effortlessly in a headwind going uphill, like being in your living room. And we got 18 mpg on the trip, better than the 3.8.
Whether it be a Hemi or LS, the V8 simply transforms the JK into a different driving experience than any V6, the sound, torque and linear power delivery. There is a cost associated for a V8 swap and that is the main constraining factor for most guys(and girls) form doing it. Our V8 swaps start of in the high $13k range for a turnkey package.
It is important to decide what you want before you choose any option, we have removed 5 SC's recently and 3 Hemi's to do LS swaps, that is a lot of spilt milk. The SC and Hemi work well when done correctly and the expectation matches performance. The Hemi has no performance issues, it is powerful, some of the Chrysler transmissions bog it down although the newer 5 speeds are better. The 5.3 will downshift a lot in a heavy JK in the mountains under load, so if you have a heavy JK get a 6.0 or 6.2. The Hemi and LS drive differently, the LS more relaxed, the Hemi pumped up; in the end they are both powerful. Driving a SC JK requires a change of driving habit, getting to know the non linearity power delivery. There are distinct power zones and combined with transmission downshifting it can be challenging to get smooth power delivery. You get accustomed to it after a while and how to work the throttle pedal. A SC V6 sounds different than a V8 too, with loud exhaust it can be annoying, it never has that V8 rumble.
In the end get what makes you happy. There are lots of SC's, Hemi's and LS's out there with happy customers. Drive a JK with what you intend to buy, compare. To invest a lot into power adders then go a different direction is a wasted effort. If you are on a budget you don't have as many choices. Look at tuners, gears and SC's. Do one thing at a time to measure gain. Gears have the biggest return on investment in my opinion with a small engine. Once we regeared that YJ I could get on the freeway and not be in fear of being run over.