A lot depends on (1) the space available for it, and (2) your budget. The Accuquilt Go! is very compact for storage purposes and does not need a dedicated space. The Studio is heavy so it needs to be set up in a dedicated space so you can leave it up. I believe the Sizzix Pro is similar in that it does not fold up for storage.
How expensive an individual setup is with all the accessories depends a *lot* on how many dies you purchase. The Studio is the most expensive, especially considering the cost of the dies; however, it also cuts more layers per pass (depending on the die you use) and with adapters can take the Go! dies. For cuts you use a lot, you might want to pay extra for the Studio dies that cut more pieces per die (as well as cutting more layers) as it makes cutting much faster. For dies you might want to use only occasionally, you can purchase the less expensive Go! dies and use an adaptor (which you also have to buy).
Some people like the Sizzix Pro because of the wider choices of dies -- you can use Sizzix dies (and I think they have more of the delicate applique fusible types) as well as Go! dies. I don't think you can use Studio dies in it, though.
For cutting primarily scraps, I think the Accuquilt Go! is probably the most suitable.