A long arm kit that hangs down 2.5" right in the middle of the frame is a band aid. A cheap way (or thoughtless way) to sell a product without the effort to make a good design that tucks the mounts up so there is no loss of offroad performance due to loss of break over angle.
The benefit of longer arms is reducing the angle of the arms. Better/flatter arm angle is the goal, and the result. For the relatively short 10-12" of travel on the vast majority of XJ's, as long as the angle of the arms is good it doesn't matter how long the arms are. Once the angle is good, the next thing that affects trail performance is ground clearance. It is well accepted that t-case drop brackets are a "band aid" to not installing an SYE, because of the loss of ground clearance. It's pretty much the same dropping the t-case crossmember and loosing clearance, or installing a long arm setup with a dropped crossmember and/or mounting brackets that are below the frame. The fact that breakover angle and overall ground clearance directly effect trail performance is a no-brainer.
I have always had design parameters for my builds, one of which is that I would never do a modification that decreased performance. Mods are meant to increase performance, so why put out the money and effort to add performance in one area but loose it in another. Sorry guys, but long arm kits that mount the arms below the frame always have been, and always will be lame. You'll notice in the last number of years all of the home fabbed long arm setups tuck the mounts up inside the frame, because that's the right way to do it.
So, if I had to make the choice between a LA kit with arms mounted below the frame, or drop brackets, I'd go with drop brackets. If you're trying to go cheap, it's a real no brainer.
BTW, both of my rigs have mid-arms.