One thing I can tell you right off is if you buy them from Schnitz you are paying more than you should. Schnitz has a good selection of parts, but you pay dearly for them. Also, don't forget that Muzzy makes lowering links.
As far as construction, they are all about the same. It looks as though most people are going with the 3 hole lowering link, which allows for adjustment, but you still need to remove the link to make the change. Both Schnitz and Muzzy offer these. I personally think the Kennedy adjustable lowering link is the best on the market. This is not the standard 3 hole unit.. it has the ability to be adjusted while it is on the bike with only a wrench. Each full turn of the link increases or decreases the height of the bike by 1/4". So 4 turns would equal an inch.. it allows for adjustments up to a full 3 inches.
Believe me, it is no fun moving a non adjustable lowering link.
I paid about $100 for my fully adjustable Kennedy set. More expensive.. yes, but definitely worth the cost.
I have the muzzys 3 hole, I was wondering about the difference in the adjustable links. I have the muzzys in the lowest link and my bike scrapes alot. Schnitzs runs 114.00 for the adjustables, I didn't know what the Kennedys run and if the material was the same. I ordered a JMC swingarm and fiqured the adjustable linkage should go on the same time.