When deciding to use LED modules for a new sign or retrofitting an existing sign, there are a lot of factors that go into choosing the right LED for your sign.  Some factors are the sign depth, how the sign will be illuminated (face or halo, etc),  and what type of material the face is made from.  Another factor is deciding between constant current and constant voltage.

A constant current driver will vary the voltage across the electric circuit in order to keep and maintain the constant electric current.  No matter the variation in voltage, the current delivered to the LEDs does not change.  There will be almost no variation in brightness across the sign and they will remain steady.  Constant current drivers and LEDs usually have a fixed output current in mA.

A constant current driver must always have the DC voltage remain constant or at the rated DC voltage of the product.  Most voltages are 12VDC or 24VDC and have a fixed output voltage.

It is most important to make sure that both your LEDs and LED driver are either both constant current or constant voltage to avoid inefficiency or damage.  Check out this video, Constant Current & Constant Voltage LED's, to see the difference between the two.