wow, nobody here knows shit about jeeps.
first of all a cherokee and a grand cherokee are two totally different vehicles. a grand cherokee is much heavier, is larger inside, and usually has nicer features, but you can get a cherokee with leather and all the accesories just as you can get a grand cherokee that is a complete stripper. First of all you need to make a decision which model you want to get. If you opt for a cherokee, look at 1998 or newer. before 1998 they had fiberglass rear tailgates that never seemed to fit or close right. also the interior got a major upgrade in 1998. If you get a cherokee get the 4.0 liter straight 6. I dont know if they were made with the 4 cylinder in the earlier years but at least after 1998 i dont think you have any motor options. I would say gas milage for a cherokee is going to be around 14 city, 16-18 highway. but it varies greatly with the miles on it, and especially if it has been lifted and is running bigger tires without a regear.
Ok so on to Grand Cherokees. They started producing the grand cherokee in 1993, and this model designation is a zj. You can get these with the 4 liter straight 6 or a 5.2 liter V8. Both motors are good, but dont think that you will get better gas milage with the straight 6, because this is not the case. The 6 cylinder has to work harder so therefore is not as efficient as the V8. There is no reason not to get the 5.2 liter. Its a great motor, mine has 170,000 and is not burning any oil, has good compression and is strong. So the zj ran from 1993 to 1998. the best year would be 97 or 98 as they had some nice updates in these years. Also within the grand cherokee there are 2 main trim options; limited and laredo. typically the limited has many more options and is nicer if you like that kind of shit, but you can find laredos that have the same things as limiteds. the limited has painted cladding while the laredo is plastic. another option to consider is upcountry. this is a suspension/offroad package that gives you tow hooks on the front, skid plates, and .5 inch higher springs. I would definitly put this on the top of the priority list, but thats just me. also i woud look for one with a factory towing package. Another thing to consider with all these jeeps in the tranfer case. and i am not going to go into all of this but if it has quadratrac IE a 249 t-case, when you test drive it make sure there is no binding or noise with sharp turns, becasue thus may mean your VC (viscous coupler) is out on you t-case and this means about a 700 dollar part and lots of hours of labor. the one other option for a zj is to get the awesome 1998 5.9 Liter V8 grand cherokee. these have a 360 cubic inch v8 that really makes a lot of hp and torque. I would highly reccomend this jeep if you want to go fast and burn gas. these are kind of hard to find. You can tell its a 5.9 from badging, rear exhaust tip is 3 inches in diameter and chrome, there are hood vents, and the lower rocker panels have a curve.
So if you want a newer grand cherokee the next ideration began in 1999 and went through i think 05 or 06. This is called the WJ. these are great vehicles, however the 1999 ones had some problems, due to it being new. the 2000-2002 model years are the best. after 02 Diamler Chrysler began decontenting it, stripping out minor things to save money. some of these things are steering wheel music controls, factory cross bars on the roof rack, etc etc. Even my 98 has these things and a 04 will not. The wj's came with 3 engine options: straight 6 4.0 liter, 4.7 V8, and a 4.7 High Output V8. the 4.7's are great engines, decent gas milage and i would highly recomend a wj with this motor. the 4.7 gets better gas milage in the wj then the 6 cylinder does. The 4.7 liter is smaller, more efficient and more powerful than the 5.2 liter V8 from the zj. however after towing large trailers with both i still feel the 5.2 is a better towing motor.
After the wj comes the wk, which is a fucking joke. It has independent front suspension, is super plasticy looking inside, and is a mall cruiser. I would not reccomend these if you are going to wheel it at all. this is the point where diamler chrysler started talking about heritage instead of actually making good vehicles.
there are so many more things to talk about and more specifics, axles, t'cases, gears, stereo systems, etc etc, so try to narrow your decision down and then we can help you more.