This is probably a matter of personal opinion, but I think vanilla is better for you than chocolate because it simply tastes better. Chocolate has more sugar and tends to be higher in calories, while vanilla is a refreshing flavor. However, chocolate does have health benefits like lower heart disease, so you should just have both!
I don't think anyone here is seeing what both of them actually are. Vanilla comes from a vanilla pod which doesn't really have that much nutritional value, its not bad for you but its not good for you either. Cocoa beans on the other hand have had so many studies done on them, that there is significant evidence to prove cocoa is good for you. Cocoa is full of antioxidants, it balances out hormones which is perfect for teenagers. I am talking about 100 % chocolate that is raw coca, if you add sugar and preservatives to everything then of course it is going to be bad for you. If i had to eat a crap filled candy bar i would pick chocolate over vanilla because at least I'm getting some nutrients. Vanilla doesn't really have a strong flavor so the companies would have to put in synthetic flavors to make an actual taste therefore making the bar more unnatural and harder for our body to digest. So if we are not talking about the pure forms or we are talking about the processed, chocolate wins on both sides.
I dont think vanilla nor chocolate are good or beneficial for you in any way. I think they are both unnecessary sweets that should only be consumed every so often. Neither have any health benefits and dont do anything for you. They are simply a treat that you should use to reward yourself with.