Which better for design posters photoshop or illustaror?

For posters and all kinds of print jobs it is better to use Illustrator as the vector graphics generates high resolution and clear prints, especially if there is a large amount of text. Photoshop can be used to prep, edit, add effects etc to the images for the poster and then imported into illustrator and compiled for the final output. Illustrator is also helpful in maintaining smaller file sizes while exporting as PDF without compromising on the quality and resolution. Another good software to use along with the above two is In Design.

If you had to choose only one it would be Illustrator. Higher resolutions, smaller sizes, and better quality in general. However, for a maximum quality and effect, use both.