I have an AGR box and it's excellent. Whichever way you go, consider other factors. I also have universal ball joint fittings instead of the rag joint and a DD bar steering shaft. It tightens things up even more, although there is considerable road noise transmitted to the steering wheel while driving and you can feel the steering box gears working on the wheel when parallel parking. It's strange at first, then quickly becomes unnoticeable. Big improvement.
Also make sure your tires are good. I chased a highway wander for a long time, had tires that seemed to be in good shape with good tread and wear patterns, but then the wander went away when I switched to another brand due to the present set being out of date.
Correct alignment settings are crucial, and by "correct", it's not what "The Book" says. The factory specs are for bias ply and for a time when highways were a new thing and far less common. They are terrible for radial tires. What you need is nearly opposite of the The Book. Make sure the shop you work with knows this and is willing to deviate from factory specs. Many shops have never heard of this and some are unwilling to do it when you ask or will just resort to The Book anyway. The specs you'll need is as much positive caster as possible, although factory control arms will be limiting. I was able to go from 0* to 3.0* positive on both sides, and the highway control improvement was tremendous. Even more caster, up to 6* is desirable, but I think you need aftermarket control arms or offset shafts to get there. Camber, I believe, should be 0* to -0.5*, and toe in should 1/16 each side for a total of 1/8. I'm pretty sure those are correct, but double check it. There are many threads on here with the numbers.
Other than being lowered a couple of inches, a 1 1/8" sway bar in the front, 7/8" sway bar in the back, boxed rear control arms, and Monroe Sensatrac shocks, the suspension on my Malibu is stock (rebuilt relatively recently with rubber bushings). Nothing crazy or expensive, just mild upgrades and it handles pretty much as well as a new car. I am due for some new springs, but that's going by ride height not drivability. It's not as soft going over bumps, of course -- it's a GM A body -- but I have no trouble keeping up on mountain roads with present day performance sedans such as Audi's, BMW's, or Mercedes. For a bigger car, the handling is excellent.