All of the umax coils should work on the silver, so it really depends on what you want to do. If your silver came with the 8 inch coil, the the 5.75 inch would help in really trashy areas and the 9x12 would give you a little deeper reach.
On a new Silver uMax it will find coins down to 9 or 10 inches which puts it in a class with the Explorer SE, GT, etc., plus the Tesoro Tejon, etc., etc.
You need to slow down and remember it has a cone shaped signal. At extreme depth you're only covering an area of about 4-5 inches in diameter. So overlapping is a must.