This depends on the application.
RGB Pixel strip uses a 3-in-1 colour (red, green and blue) LED IC. By having the three colours, red, green, and blue, the end-user can obtain a nearly white colour at full brightness. RGB pixel strip also uses three channels per led, which means fewer DMX universes are needed for control.
RGBW Pixel strip has a 4-in-1 colour (red, green, blue and white) LED IC. Although RGB can produce a colour close to white, the dedicated white LED of the RGBW pixel strip provides a much purer white tone. RGBW strip uses four channels per LED, which requires a higher number of DMX Universes for control.