Steeda is overpriced in my opinion as well. UPR, from my understanding, just makes crap products. The only thing I've ever had from them was a clutch quadrant, and I haven't had any issues. However, I've heard tons of horror stories.
Maximum Motorsports are hands down the best. They are true innovators in the Mustang suspension world. These guys really know what they are doing, and they prove that with the cars they have built for the track. You pay more for their stuff, but it's because the parts are actually well engineered and well built.
And no, I'm not on the MM payroll, but I wouldn't complain if they started sending checks my way for promotion.