the brunette is far more natural looking, it is not natural to have such blonde hair unless you are a young child or albino! when work collegues are forming initial opinions of you they are more likeky to 'trust' the brunette as it is obvious the blonde is hiding things (the real colour of her hair for one thing)
if you are looking to pull a guy however, most prefer a blonde.

personally you look beautiful whichever you decide and any guy worth his salt would see past the colour of your hair and see and be attracted to whatever is on the inside of you. just steer away from those attracted to your blood, they are vampires!