My Sahara has both, I am not sure how they will work together.
If you off road, you need the limited slip, it is expensive to add later.
ETC is not for off road conditions, it will prevent you from applying power to the rear wheels, and possibly front, not sure.
In mud the ETC will fight to prevent the wheels from spinning, and as you apply power it applies more brakes. It is great for on road conditions such as sand, snow or ice on the road. I used it once to get a Yukon unstuck, but could only apply a little power and rocking motion to get it out. If I had used a lot of power it would have burned up the brakes.
The limited slip links the rear wheels so that they rotate the same speed. I think the explanation of applying different amounts of power to the rear wheels is misleading, it only locks them together. An open differential would allow the power to go to the wheel with less traction, since there is less resistance. The limited slip delivers equal to both wheels, so if one wheel has traction, the power can be delivered to that wheel, and if the wheel with poor traction is not spinning it has better cohesion.
If you don't intend to off road, the ETC is enough, the limited slip can also cause extra wear on the tires.