Obviously there are some differences in the effects of Xanax & Valium, since they're different chemicals. But, for the most part, I'd say it's safe to say that they're more or less used for the same things & have the same effects.
&, I don't know how many other people who reply to this will or won't agree with me [& I don't really care, for that matter], but I'd say it's also safe to say that overall, yes, Valium is weaker than Xanax. Of course, when you talk about one drug being weaker than another, there are many ways that could be interpreted. & I wouldn't be the least bit surprised if SOME people experienced stronger effects from Valium than they did from Xanax, & I wouldn't be surprised if there were at least one or two very specific & small ways in which Valium was more effective than Xanax. That aside, though, I'd say Xanax is stronger than Valium. In mg per mg potency of course [.5 mg of Xanax is roughly as potent as 10mg of Valium], but the main reason I'd say Xanax has significantly stronger effects than Valium is because Valium almost always comes in dosages of 5mgs or 10mgs. Xanax comes in dosages of 1mg pills most of the time... It also comes in .5mg pill dosages & 2mg pill dosages [2mg Xanax pills = "Xanax Bars"], but it seems like without question more people get 2mg Xanax pills than .5mg Xanax pills.
... However, 5mg Valiums are equal to .25mg of Xanax, & 10mg Valiums equal .5mg of Xanax. But the common starter dose of Xanax is .5mg [or sometimes 1mg], & the common starter dose of Valium is 5mg. The common recommended dose in general of Valium is 10mg & the common recommended dose in general of Xanax is 1mg.
So, incase you haven't caught on, that kind of means that the way both of those medications are prescribed & such makes it so people on Xanax are generally experiencing more or less twice as strong an effect as people on Valium are.