The 2 function completely different.
The cathedral entrance is front and center. Cannot miss it.
Always a line but no tickets, admission is free and moves fast, is basically a security check.
There are at times 2 lines ; at least there will during mass times ; one is for mass attendees and that usually has a 1 min wait and another for tourists who wish to see the cathedral but not attend mass and that usually has a 10 min wait.
Been for mass a couple of times.
The tower is on the side ; left side if you are facing the cathedral and the main entrance.
That used to work on a first come first serve basis and you have to purchase tickets there, no advance purchase possible.
However recently they added this app ; called Quacky Duck or similar which only works when you are in the vicinity ; but that allows for advance ticket purchase for a set time ; only same day tickets though I believe and will only allow you to buy tickets when you are within a block or so.
If you use that app you can avoid any lines ; no cell connection you have should show up early and wait on the line as I imagine the new app makes it worse for those not using it.
Basically the app is intended so you go to the site, buy your tickets for the tower and then site see at the cathedral until your time for going up the tower is ready.
Have walked up the tower but was a little less than 2 years ago before the app existed. Recently was there and downloaded the app but decided to skip a return stair climb.
I tried using the app not far away but across the Seine about 2 blocks away and it told me I was not close enough.