Welcome to the forum.
I've had Safe T Plus on my rig for over five years and love it.
I think you might be confused a bit about the SS you mentioned. Blue Ox makes Tru Center and it's main feature is the "adjust of the fly" capability. It's also available with the STP, but it's an after market add on trim kit.
When I had my STP installed, Blue Ox was having a lot of trouble with the quality of their Tru Center but by all accounts these have been fixed and it seems to me that the Tru Center is now probably the preferred unit by most members.
Feel free to use our search feature and you'll find a ton of reading material on the subject.
Best of luck.
Rick, Nancy, Peanut & Lola our Westie Dogs & Bailey the Sheltie.
2007 Itasca Ellipse 40FD