With the Accuquilt, you do not need to pre-cut the fabric before running through the machine. Excess fabric just rolls under the machine with the die.
With the multiple shape dies, you place fabric layers only on the die cut you want; you just don't place fabric on the others on the die. In this case typically you would fan fold the layers and finger press the folds before placing the fabric on the die.
What I find the Go! really useful for are triangles (the die pre-cuts all the corners so piecing is very accurate) and other odd-shaped pieces such as hexagons and circles. It is also really nice to have for certain blocks, such as Hunter's Star, which are difficult to cut and piece accurately. I like to do freezer paper applique sometimes, and the Go! dramatically speeds up the cutting of the freezer paper templates. Many people like the strip dies, but I prefer to rotary cut strips myself using my June Tailor Shape Cut mat.
There are many Youtube videos you can watch that will give you an idea of how the machine actually works.
I don't know much about the Silhouette; I think it is more for cutting your own applique ideas out accurately, one at a time. The Go! is made by Accuquilt and comes in two forms nowadays -- manual crank and electric. I have the hand crank but would get the electric if I were buying now (users who have both love the electric). Accuquilt also makes the Go! Baby, which is smaller and the Studio (sometimes on sale for $395). I would love to have the Studio because it cuts so many pieces at one go, but you need dedicated space for it as it is a heavy machine.