It's slightly more expensive to run in my experience, but not much. The big block torque gets you on plane with less work, unless you keep adding weight. The boat will take as much weight as you want to throw at it, which was our issue. We kept making the wake bigger and bigger and bigger. The Xstar with the 8.1 is still probably one of the best wake machines ever. Unlimited power.
The downside....... 8.1 has a history of blowing head gaskets, sucking water into the cylinder, and hydro-locking. We blew up three 8.1 motors in less than two seasons. Be sure to ask the owner for records and ask if they have had engine trouble.
I had a 2005 X2, and I didn't think the Xstar was much more expensive to run, if at all.