I would say a pressing comb, because to me, it does a better job. If you use a flat iron, it never gets inbetween the strands, and you can't really press your roots. I know a lot of people that press their hair with a flat iron, and within days they have to do it all over again...what is the point. When I get my hair pressed, it lasts 3 weeks...that's when I wash my hair.
Yes, the flat iron is easier, but does it make you press stay as long as if you were to do it with a pressing comb? It's better to be careful with the hot comb, so you can get a complete press, than you a flat iron, then to have to do it all over again in a couple of days.
Hey...why not try both, and see which one holds up?