I've got the stock size full set (crank, power streeing & altenator).
The underdrive pulleys are smaller (even lighter I think), remember a different size of belt would be needed for these.
I heard that the underdrive pulleys are not so good at working the altenator, therefore short runs in the scoob would eventually leave you with a flat battery. I do not know if this is true or not (so dont quote me) but I decided to go for the stock size anyway.
I never really noticed a difference after I got mine fitted but as Higgy says, all those little things add up.