In most cases, the improved design is something that Moog found to improve their design. It is only listed while the supply of "non-improved" design exists. Once the old stock is sold out, they no longer have an "improved" design, it just becomes the regular. That is most often the case. Now on things like king pins, they will have nylon and brass and they will always offer both.
As far as what it is, probably heavier design. Moog generally has grease fittings on all their units. Moog also makes a couple of different private label products, and they generally are boxed for like AutoValue, NAPA, etc. and they sometimes are a lower cost alternative and almost always didn't have grease fittings to save cost.
Ask them to get you both. Any parts store worth it's salt will get them both in and let you compare. Personally, I would buy the one that looked best, which I bet you will find to be the "improved" one.
