I prefer seated, but, I know there's plenty of smart people on here who would disagree with me. Truthfully, the only way to sufficiently develop your calves is to do both.
More importantly, why are you buying a calf raise machine, of all things? This indicates that you probably work out at home and have a home gym of some sort, in which case you can just do all the calve work you need with barbells and dumbells!