Both shows exist independently of one another and you don't have to watch the one to understand the other. However, I feel to better understand some of the more subtle references in "Caprica" to watch "BSG" first.
Since Caprica is a prequel, it can't spoil the ending of BSG. Nearly all of the BSG series will stand independent of the rest. I recommend going to wikipedia and seeing the order things were aired. There may be some subtle references that you will get watching it in that order that you wouldn't otherwise.