Because of the 5.25" Scan-Speak mids, the Silver Monitor isn't as robust and won't handle as much power below 150 Hz. Those are the same mids used in the Platinum LCR, but in the Platinum they're crossed over from the 10" drivers at a higher frequency. Silver Monitors work great in smaller theaters where very high output isn't demanded. Also, with a 4"-deep InWall version, they're a problem solver for installations where you don't have the 6" of stud space for the InWall Gold/6 Monitor or InWall Gold/6 LCR. And there's also a great InCeiling Silver/6 Monitor, which may be the best ceiling speaker for LCR use.
The sound quality of all three is comparable, with the Silver Monitor being a little lighter below 120 Hz or so, but the two Gold models play louder and are more dynamic. If you have the room, pick the Gold LCR over the Silver Monitor, which are both around the same price.
It comes down to the appropriate speaker for the application. That's why we have 100 different models.