OFDPOS,
I don't know much about the STP. I remember that when I was making a decision it did not was of my liking. In addition, it was more expensive. I like the spring used by Roadmaster which is not part of the STP. So, I decided for the Roadmaster. It took me less than an hour as my friend handle one side and I did the other. It has three bolts provided for the frame at the passenger side. I am glad I decided for this one instead of STP.
If you have questions on the Roadmaster, I will be glad to answer. For STP questions, you need someone else.