While making arrangements at school last year for Horticulture Exposition Contest we could choose real or silk. I chose silk both times I've done the contest. One thing I loved about it is you can still make a very very nice arrangement that will last a long time, you can find very nice silk flowers. Some are very realistic. I also enjoyed being able to make it weeks before the contest and not rushing to get it done! Plus silks are cheaper in many cases.

Also I would choose silk because it prevents early wilting and things like that. Silk is better for any occasion over fresh IMO.