In my experience:
The $100K in the 529 is expected to be used 100% for college.
The $100K in the brokerage account is expected to be used at a much smaller %.
Edit: If the $100K were in a brokerage account that was part of your 401(k), it would be 0%.
They are absolutely treated differently. Very differently.
The tax benefits of contributing to a 529 are one side of the equation. How money is viewed in determining need is the other.
Each institution will view things slightly differently, but those two accounts are, in fact, viewed quite differently.
As you plan your future, the first priority, and first dollars, need to go to your retirement account. Then, once you are funding that at the proper level, you can consider a 529.
When my oldest child was eight years old, I started a 529 for her and her siblings. The next year, my company went into bankruptcy, and my pay was cut by 65%. I was struggling to pay the mortgage, so college would simply have to wait.
It wasn’t a matter of planning for me, it was the reality of very difficult times.
But, coming out of that, after I’d gone back on active duty, when paying for college, it was very interesting to see how colleges viewed things.
I sat in the office of the Director of financial aid at an elite school, where we discussed my financial situation, as well as their offer for my oldest daughter. That conversation, as well as the six college educations, for which I have paid, has shaped my comments here.