The 2MP UltraStarlight will give better quality than 6MP Ultra Non-Starlight.. Same sensor size with less pixel density = better low light capabilities.
Thats the IMX178 in the 6MP, its rather good sensor.. probably one of the better UHD sensors, but more megapixels dont mean more detail and better quality.
Ultra UHD 6MP = 0.02Lux/F1.53(Color), 0Lux/F1.53(IR on)
Ultra Starlight 2MP = 0.002Lux/F1.53( Color), 0Lux/F1.53(IR on)
Thats an order of magnitude better low light capabilities.. but the UltraUHD is likely to be one of the better UHD performers if your really after megapixels for some reason.