Actually, the CPU slot is not much more than a CPU socket in slot form (in the 2000A model it is exactly that). There's shouldn't be any performance difference with the same accelerator design.
The slot form is mechanically more stable and has the advantage of using a dedicated slot space instead of potentially interfering with other stack-on expansions (A500 users know what I'm talking about).
The socket form has the advantage of fitting into an A500 as well, or possibly even an A1000.
