That's not certainly true. You can use Bootstrap at any level, even, skipping every of the baked modules and make a website who doesn't look like Bootstrap.
For the variable parts, thats another myth: you can override variables easily.
I have nothing against Foundation, but to me, all the frameworks claim silly things to gain developers and developers aren't that kind of stupid consumer.
Just bring the facts, let the devs decide.
Bootstrap's eco system is huge. If I had to use one, it would be Bootstrap. Since I can choose either, I use Bootstrap most of the time and Foundation when I need more control.