I think the main difference between the SD arms and the fbi arms are the materials used. I know first hand that the SD arms are built very very beeefy, extra thick materials. The SD arms also have a custom machined x-shaft with them which is very very nice also, and is greaseable. And another reason, no matter how you feel about it, is the name. When you buy a SD product your know right off the bat your getting quality, and great service. Im not saying that FBI arms arent great quality, and dont come with great service, but its been shown in the past how good the service from SD is.
Another option for you would be the arms from
www.barelylegalfabrication.com . These arms are also going to be sold through SD , and are very well built also. They come with a Uni-joint upper pivot that replaces the balljoint, and the ugly bulky ass plate the sits in. They are also built with very thick and heavy materials, and also come standard with the SD xtra beefy machined x-shaft. These arms are what i personally run, and i love them, ive never had a better control arm.