I have had a lot of different K members stock to Team Z to UPR, MM, etc.
The stock one works fine with 5.0L and most 5.8L V8 swaps. If you must put an aftermarket one in for whatever reason, MM is the only one I actually like. It is a little lighter than the stocker but not by much.
Regardless of which one you choose, be aware that most of them won't accept stock control arms and stock rack location. most are "improved" such that you MUST change to coilovers (no spring pockets), tubular control arms, and then you'll have to address bump steer and sometimes even some of the aftermarket K members move the control arms forward/outward, then you get into tire/fender clearance problems. This is one of those situations that one must plan for, research highly....because like what I went through, if you don't, you end up with several thousand dollars of necessary changes just to make a little more room for headers and starter....
The MM k member is a little heavier BUT it is also a LOT beefier than most others. They actually test their stuff....and the customer service there is top notch. I had a QA1 K member in a fairmont once; bolt holes were off a little, called the company to discuss, they said "that's the nature of aftermarket parts". I got it in, not easily mind you, and decided not to buy their products anymore. Customer service goes a long way; even how the first person you call talks to you...that is your first impression and if it's not good, the company's image as a whole is damaged. And for those that work for a living...keep that in mind....