Can a drug test differentiate between a 0.25 mg or a 2 mg xanax?

It depends on when you took it (the xanax). Xanax is out of your system in 3-5 days TOPS. It is water soluble, and can be out even sooner (1-2 days) if you drink adequate fluids (not even a lot) where Valium is not. Valium will stay in your system longer than xanax. If you only took that small amount, you probably don't even need to worry about it.