If you plan on a lot of highway driving and/or you want to run much bigger tires, you would probably prefer the extra power of the 4.0 inline six that was used all the way through the end of TJ production which ended in 2006. The 2.5 four cylinder, which is just a smaller, less powerful version of the 4.0, was used through the 2002 model year. It is great for off-road or In-town driving as long as you don't run much bigger than a stock size tire. Both engines are extremely durable and have good low-end torque. The 2.4 four cylinder in the 2003-2006 TJ models is a different engine than the AMC-based engines mentioned earlier. It makes more horsepower than the 2.5, but I can't speak about the durability of that one because I have never owned one. If you can decide what you will be doing with the Jeep (lift, tire size, other planned mods, where you will be driving it, etc.), it will help you decide which engine you would prefer. You can usually buy a four cylinder TJ cheaper than you can a six cylinder. They might get a little bit better gas mileage than a six cylinder, but not a great deal better. Jeeps don't get very good gas mileage anyway, so just be aware of that from the start.