I have used both 5000°K and 6500°K bulbs. Some definitions I have found:
5000°K "Horizon daylight" - Cool White fluorescent or CFL "daylight" bulbs.
5500°K "Vertical daylight" - sometimes described as high noon in Denver
6500°K Daylight, overcast.
Personally, I prefer the 6500°K bulbs - more white - but the 5000°K CFL bulbs are much more commonly available - and usually cheaper.