There's a big range of digital signage systems available these days, the most basic choice will be do you require an interactive touch display or a purely visual one only showing videos and pictures or other information. For interactive displays you will require a PC or Android supported display.
Another important choice will be on how the display is updated, will you be showing rarely updated content or content that changes often and needs remote management. If you are displaying just a sequence of videos or pictures then a simple display with USB support for the data may suffice. Some displays that offer remote content management will require a monthly subscription with the display provider. Though the fees are low considering the benefits and ease of management that you get from using such a cloud connected display.
There is also the form factor to consider, do you want a floor standing kiosk type display or wall mounted signage. If the display is for use outside will it be exposed to the elements? can the display be read in daylight? All the displays we review here are for indoor use only.
You may ask yourself why some of the digital signage displays offer less screen resolution than a similarly priced TV, or why they cost more in many cases. These commercial signage displays are built to higher specifications to with stand constant use in a commerical environment, hence the premium cost.