In simple words, you don't want your pic to be bit of blurred out with 1080p resolution on 27inch screen. The amount of pixels in 1440p resolution are more than 1080p resolution thus giving you better picture quality. Things to look at before buying monitor are its resolution according to the size, response time, refresh rate of the screen, contrast, panel and i would say even reputed brand for which you are going to put money for good.
If you are hardcore gamer, going for 3D and love having smooth experience from your display, then Hz does matter. If normal user then you won't have problem with 60Hz.
Note: For 3D, 120Hz is must.