Although I love Saga ships I think the accessible cabins onP andO are in some ways better planned than Saga, furniture wise. However Saga do have excellent nightlights and automatic doors. Both have shower curtains but very little storage in the bathrooms. There are slopes near the window and on the balcony on Saga, presumaly for the wheelchair, but I find them quite difficult to negotiate.
Saga have many extras, big bottles of toiletries, slippers, bathrobes, shoe horn, binoculars and a blanket in case you are cold on the balcony. Also 2 large bottles of water a day and more if required.
Both have tea and coffee making facilities.
Both ships are adult only, but Saga are all inclusive, but more expensive.
I’m sure you will enjoy the cruise, whichever line you choose.