Where do you live?  In most areas, neither is necessary and the dealer stuff
can actually plug drain holes and damage the car.

A loaded question! The answer? BOTH. But not if the dealer is selling the
white gummy stuff as opposed to the solid black stuff.

I live in Southern Ontario. Rust is something I have had to battle for
every one of the 24 years I have owned my own vehicles.

The very best rust treatment--bar none--that I have EVER come across is the
"drippy stuff", Krown or Rust Check. However...

Either is basically the same stuff, and either MUST be reapplied at least
once per year, at a cost of about $110Cdn per year.

Personally, I use Krown. I used to use Rust Check but was not happy with
the guys that run the franchise near me. The Krown guys seem better.

Several caveats:
1) It's drippy and messy. Get it done before Nov 1, or whenever your
municipality bans overnight street parking for the winter. Leave it on the
street for at least two days.
2) It will swell your door seals and any other rubber weatherstripping on
the vehicle. This will take several years to show up, but it will swell
them. Swelled seals are OK for a while, but eventually they will start to
fall off the car. The only cure is replacement, at between $30 and $100 per
seal.
3) It cannot protect where it cannot be applied. Don't count on it to save
you from those zillions of stone chips you will get every winter if you
drive any significant amount. Don't count on it to protect you from rust
around the windshield frame, door tops, undercarriage, or any other place
it cannot be applied or gets washed away. Don't count on it to save you
from rust in those areas where sand and grit blast away paint: that's where
the black stuff is needed.
4) To help combat #3, go to Canadian Tire. For $8 you get a can of aerosol
Rust Check. Do your own touch up in those areas that the franchise cannot
do.

It's the best. The very best there is. Because it stops rust from occurring
in the worst places it can happen, the places nobody ever sees, the places
that kill your car.

If you want more detail, post back here and I'll oblige.

Isn't 'rust protection' something that has become known as something
useless that the dealer makes bucks on?

I mean, what do they do?  I remember years ago, 2 decades, it was a
'wax' sprayed into the hollows of the doors and frames.  Is that
necessary today with fewer metal parts, and better paint/paint
techniques?

later,

tom @ www.CarFleaMarket.com

I have a '91 Celica with 191,000 miles on it.  I never show it any
respect, and it has no rust on it (nor any add-on anti-rust treatments).
 I live in central Ohio where road salt is applied almost daily for months.