Teenagers have been getting drunk on alcohol since the dawn of human civilisation many many thousands of years ago but xanax bars have only been around for less than 100 years so i reckon the teen drinker would have a healthier brain development as the times i've tried xanax it made me relaxed but turned my mind into mush and significantly reduced my cognitive skills as well as wreaking my ability to communicate with coherent speech. You could say alcohol has the exact same affect on a person as xanax but the feeling of both is different and the key point of difference in the drugs is that alcohol can be used in the long term but xanax is only meant for occasional use.
As far as potential iq decreases both getting drunk and taking xanax would have to have a negative affect on a young teenagers mind but more in relation to emotional stability than intelligence.
Although to me it would be far more easy to maintain a social drinking habit of getting drunk on alcohol in the long term than building up a physical dependency to a benzo like xanax.