You will get the most informed answer if you post your layout. Sometimes one level is best, sometimes two, based upon what functions are on the island, its relationship to the great room etc.

Last remodel we had the island all countertop height and we loved it! We had one piece of granite appro 9.5' x 4.5'...which really showed off the piece of granite in all its glory.

We much more prefer just a small step up to sit at the bar top instead of the full amount. Chairs are closer to regular size. If you have a counter height dining room table, the chairs can be moved there also.

Current remodel will have the one large piece of granite also.