Here is a first hand report from the people who have actually driven or set in both Tesla Model S and Model 3. They share their opinions in a current discussion happening now in Reddit. Here are some of the most interesting quotes from them.
Filippopotamus who is a Tesla investor and a future Model 3 owner writes this.
I can’t speak for the driver seat. But I have sat on every seat of the Model S and the backseat of the Model 3. Comparatively, both backseats are very comfortable, but surprisingly, the Model 3 has more headroom than the Model S in the back. I am 6’4” and I can feel my hair touching the roof of the Model S (all glass roof). I had plenty of space between my head and the glass roof on the 3. Comfort-wise, they were pretty comparable. Again, no idea about driver seat though.
Iwantatesla, who is a current P85 Model S owner and a TSLA investor compares the seats. He writes that "the Model 3 seats feel like the Tesla Premium Seats (currently in S and X) in terms of the way they hug and comfort. The S seats are bigger though. I have been in the back seat and front seat of the 3 for multiple rides and all iterations of S seat."
Here is another interesting comment.
"A car could be the best most awesome car ever but if the driver seat isn't comfortable I won't be driving it much less buying it. To give you an example the last time I bought a car I sat in about 5 different cars. 3 of them didn't get a test drive because the seat was uncomfortable to me. I have sat in a Model S so it would seem to be a logical thing to compare the seat to much like you would compare the seat between 2 different ford or Toyota cars. I have a bad back and certain things are deal breakers and if the seat seemed way off I wouldn't even look at it and start looking at purchasing a different car when I was ready."
Overall, it's best to get back to this question in few months once there's a bigger group of current Model S owners who have received their Model 3 cars, and can do a comparison based on more than a 15 minute ride. By then, the Model 3 might even be in retail stores for you to sit in yourself.