I'm sure either would work.
I have a preference for all buttons equal to the longest text. Truncating text kind of defeats the purpose of showing what it will look like in real life. Yes, Balsamiq isn't used for high fidelity mockups, but I often want to show folks how foolish an idea might be in wireframes first, so they go and find a shorter name. That and there's rarely a case in non-mobile sites where you'd truncate instead of extending. Since desktops are much wider.
However others who mainly do mobile design may have a different opinion. But I'd hate to add onto the feature and make it too complex by giving us both options.