PHOTOSHOP OR ILLUSTRATOR FOR POSTERMAKING

It all depends on the style of art you're doing, sizing and a few other factors and some may require the use of both programs. If it's all hand drawn or needs to be on a very large scale - go with vector art in Illustrator. If it's a lot more intense graphic wise and you need a lot of effects - go with Photoshop but make sure your file is set up properly in terms of dpi, sizing, etc. Most print shops prefer things being formatted through illustrator though.