I've had both. I had a 5.9 that I bought with 20K miles on it in 1999. I can say it was the fastest and coolest piece of junk that I've owned! I've had about every flavor of Jeep and the 5.9 just wasn't designed to stay together. When the transmission started to go at 35K it promptly got traded. The sound. luxury, and muscle still call my name at times but when you read about rear-ends, transmission, transfer cases, etc there's no way I can afford to keep on on the road, not to mention all the power and electronics that are quickly aging. Finally, you are supposed to put 91 octane or higher in them which is unreal these days!
My current daily driver is a '92 MJ and my wife's is a '97 XJ and even with the quirks each has, I consider them reliable and something I can work on and averaging 19-20mpg in each isn't bad.
Just my opinion! :)
Brandon