I would never walk Daisy on a collar. Chis have fragile trachea which can collapse if too much pressure is applied to their necks, which can be done by them wearing collars.
With harnesses, the pressure is spread out more along their chests.
I use the fabric step in harnesses, they are soft on Daisy and she likes them.
I prefer a harness because of the delicate trachea. A harness is also handy if you get into a situation where you need to get your dog out of danger quickly. You can just pick the dog up by the harness easily and without hurting it at all.