I too prefer Sinatra by a wide margin; especially a young Sinatra. Although neither as much as Bennet, but that’s a different story. Sinatra’s phrasing was impeccable and considered by many singers and instrumentalists as “the standard”. The particular quality of their individual voices is a matter of personal taste and although I don’t particularly like the quality of Sinatra’s later in his career his sense of swing as well as phrasing was always pretty great; a true jazz singer. Dino’s voice had an inviting warmth not unlike Bennet’s, while Sinatra (especially later in his career) would sometimes convey for me the sense that he was doing the listener a favor by singing. Ted Gioia does a good of addressing some of this:
https://www.thedailybeast.com/just-how-good-a-singer-was-dean-martin
https://www.thedailybeast.com/just-how-good-a-singer-was-dean-martin