Depends.
If I want the molasses flavor in a sauce, in the background, I'll use dark (if I want a pronounced flavor I use molasses). For most pork rubs I use light or white or a combination. Though I rarely include sugar in beef rubs, on the occasions I do it's usually dark.
Dark can bring out flavors in some fruits used in sauces (if you use fruit in sauces), especially pineapple and banana. Honey works well here too so I often use both. In sauces or paste rubs (not in dry rubs) you can recreate the flavor of dark by using either white or light and adding a touch of molasses--if you happen to be out of dark at the time.