Better flow...usually in combo with a ported head. Depends on what you plan on running for a turbo (flow wise) and what HP you are trying to hit.
Cost...the valve seats have to be re-sized to fit the larger valves, plus cost of the port job. Highly recommend upgraded springs with oversized valves running a higher lift cam.
When planning all this, you have to look at what the turbo is going to flow in CFM. Your IC & piping, TB, manifold, and head need to be "matched" to support the flow.