I am with @Frederic on this one.
I am assuming you meant the Jeep JK vs the JL, the later being the newest version of the jeep.
It would really depend if you will be using the jeep off-road or just for daily commute. In a few words 5 speed is better off-road and the 8 speed is better on highway.
The 8 speed claims to reduce fuel consumption by 9% over the 5 speed transmission but I doubt it is for the transmission, I think is because JL has an aluminum body as opposed the steel body of the JK, making the JL lighter and therefore improving fuel consumption. I think that for on-road driving will definitely benefit from the 3 extra speeds since you will run in lower revs.
However, when it comes to off-road keep in mind this 8 speed transmission is used in the Ram 1500 and Grand Cherokee, cars that are not necessarily made for off-road like the Wrangler this is a transmission that is not properly tested off-road. They are only installing this transmission for cost purposes and they are not thinking about the reliability.
In off-road conditions simpler is always better, unless you use the 8 speed in triptronic it will be under much more stress than the 5 speed.
Great question @Frederic and although i do not have any idea but the 8 speed would be much better than the CVT gears.
Moreover the 8 speed gear in a Jeep with their off-road capability i am sure the makers would have kept that in consideration and it would perform just as well in the desert.
If you do not get the feedback here, you can always test drive a Jeep in 4H and pump the accelerator to get a feel of the gear shift. I have a friend in Jeep sales at Festival City, I can pass on his name to you on one of the drives.