Both things are extremely common on supermarket aisles (even in the UK). But yet dentists suggest you avoid them. That's a rather interesting state of affairs.
Actually my dentist recommended Sensodyne (non-whitening) and a fluoride mouthwash without alcohol (the brand escapes me).
My dentist loved me. My dental visits consisted of me walking in, getting briefly looked at, the dental hygienist doing a quick go over my teeth, complementing me on how clean they are, and sending me on my way.
When I want back to eating sugar, my dental visits suddenly got a lot longer. The cleaning of plaque alone is now at least twice as long.
Dropping the low carb thing I can understand. I do low carb paleo on and off as my fiscal and lifestyle circumstances dictate... but I keep my sugar intake low regardless.
That said, you're right to have some skepticism about heavily marketed tooth pastes. For instance, I have four veneers and my dentist recommended against using any "whitening" toothpastes, because they're naturally abrasive against your teeth and can very slowly damage them over time. I just use Sensodyne and follow-up with a non-alcoholic flouride mouthwash, and I've had no dental issues since, even with a sugary diet. But I brush/rinse three times a day.