Here is a good site for reference http://www.vanilla.com/html/facts-extracts.html
I'd stay away from imitation vanillas if I could. 1. because they don't taste very good and 2. they're made from synthetics. I personally use a vanilla extract that is made from Madagascar vanilla beans and alcohol. Try to stay away from extracts with "additives or natural flavorings" because it's more than likely corn syrup, caramel coloring, etc..