Apologies in advance for the length....to a question nobody really asked!
It comes down to personal preference. Moog was "first on the scene" 5+ years ago when people started finding it necessary to replace parts in the suspension of their late B-body cars due to mileage-related wear or through trial-and-error in fixing little problems that only showed up when the larger wheel & tire setup on the Impala SS had wreaked havoc with the OE steering parts and/or bushings because we all drove the car like it was a race car (which typically receives much more frequent maintenance than a street car).
Over time--starting in about 2000, I recall, Moog came out with the "T" version ruggedized parts, which included the significantly beefier K6187T idler arm that still is the most commonly serviced steering system part on our cars. By now, however, if these parts (on "your" car) are still stock, and regardless of mileage on the car, if its over 50K, I'd be looking at all of the other wear parts in the steering system--especially the center link, and, of course, the tie rods.
Even replacing these parts is no guarantee that all will be "right", since the steering gear itself is prone to wear and mis-adjustment, so that has to be looked at with the idea that it may need some work, too--not something to do at home, for most of us, anyway. And then, there are the control arm bushings and ball joints, too....
With all that said, being one who has shared alot here, I discovered that GM, thru the ACDelco replacement parts line, was offering what I considered to be equal or superior components when compared to Moog--and while there are plenty of stores that sell Moog, it was very frustrating answering the many questions about "where do I find Moog/Problem Solver parts?". I had my own good source for Moog, but for me the answer was to tell everyone, via this Forum, that there are also parts you can buy from GM with standard GM part numbers (ie. Dal and others--and your local dealer) that get the job done, and by virtue of that fact, are "easier to get" than trying to figure out where the best local Moog source was for someone not living where I was. If you go to a GM dealer now and have them look up the B-body parts, you'll find that most of the original PN's for replacement suspension parts have been purged and replaced with the ACDelco service-line equivalent.
I have found that I am very happy with the ACDelco line of parts, which are sourced from Dana Spicer, and are the same parts sold by NAPA, and in the Spicer "Profeesional Line". I have had quite a few packaging errors from Moog (a Federal Mogul company) and with the GM parts, virtually none. Don't misunderstand, Moog makes good stuff--and their K6187T idler still has no equal. But, if you've looked at the pictures that Wayne Bengston posted of the ACDelco parts, you can see how nicely the parts are finished, unlike the Moog stuff. In general, I've found that the ACDelco parts have some features that I like better than Moog, especially the plating or painting of the parts, the polymer-lined (lower friction) full ball design of the ball joints, and in the case of the upper ball joint, it is a single part that is pre-assembled with a nice poly boot crimped into place, where the Moog equivalent comes as 3 pieces all held together by the 4 bolts that mount the parts to the upper control arm.
This is not intended as a selling job to have anyone stop using Moog. As Larry has pointed out, there are good sources for Moog out there. It is also true that some parts are identical, no matter what package they come in--as Terry points out, bushings are generally common, apparently coming from a single source, it would appear--and its not even Moog or Dana (try Clevite, which may be a subsidiary within Dana). Other examples: center links often look the same regardless of source, as do upper control arm cross-shafts (the offset version).