Yeah, but it is just a "Space Shuttle Switch" simulator, which is the difference to SSU.
That is not fair. Granted SSU is for Oribter, which means all the onus is on the user to get the launch right, the orbit right, , the re-entry timing, everything, and also there is some freedom to get creative.
SSMS does a pretty good job simulating the GPC, APUs, the RMS arm/system, the APDS, the cooling systems. It focuses a lot on those systems, some are just switches, but a lot are not. It doesn't require proper cryo configs, makes rendezvous seem a bit to simple, doesn't really simulate the IMUs and the other navigation systems.
If the plan is for SSU to have a full Genearl Purpose Computer, and not just crude mimics of OPS1, OPS3 like Shuttle Fleet, actually have OPS202 for changing the orbit, a DAP to hold INRTL and LVLH attitude, and provide the proper counter thrust for the RCS, then you are really cooking with gas.
The Shuttle is not just about removing latches, opening the payload bay doors and deploying the KU attena, or even turning on the APU at the right time. The heart of flying the Shuttle is the GPC, especially OPS1,OPS2, and OPS3, and the DAP, and if the sim has that, then you can have a pretty good shuttle experience. That is what SSMS simulates real well, far beyond anything I have seen for Orbiter thus far. Even as nice as AutoFCS was.
But I have to admit, I was not aware a version of SSU was available, so I certainly will give it a look and help out by reporting bugs.