Since UPR and Steeda are both located down here, they have displays at all the Ford related shows/races in the area. What makes the MM kit better, at least with the bolt through kit, is that they provide 2 different length bolts depending on the shim stack you need, and quite a few more shims of thicker and thinner size to get the bumpsteer closer to an ideal setting. The other kits I've seen only came with one length bolt, and the shims were fewer and did not offer as much of an adjustment range than the MM kit. I am not an expert concerning heim joints, but I would wager the heim joints in the MM kit are pretty high quality, whereas the UPR joints are probably whatever they can get to turn the maximum profit. Know anyone running a UPR bumpsteer kit in AI? The Steeda kit I've only handled once, and it appeared to be nice, but it still didn't offer the adjustment range of the MM kit. Perhaps their heim joints are good quality, but I'll just go with the MM kit and be done with it. With a cheap kit, maybe you can adjust the bumpsteer to where it's acceptable, but with a more comprehensive kit you can adjust it more precisely, and the cheap heim joints are going to crap out on you more quickly, especially on the street.
If you really want to do it the right way, find a chassis shop that can do the bumpsteer check for you and build a bumpsteer kit using one piece tubing for a spacer instead of multiple stack spacers. They'd probably get the bumpsteer setting near perfect.