These games are absolutely neck and neck. Both stone classics. I suggest owning both if you can.
I give Fathom a slight edge in gameplay just because I never get tired of it. Centaur does have inline drop targets but they aren't at all integral to gameplay. It is very satisfying to break a million on Centaur anytime you can, but Fathom demands you push your score into the 3-4 millions in order to get that same satisfaction. Centaur can get into rinse-and-repeat territory in a high-scoring game, whereas, for me at least, Fathom constantly seems to surprise with its complex ball geometry. Most people never even shoot the inlines in Centaur, concentrating on the Orbs targets and the right-side drops for points. Fathom demands that you shoot both sets of inline targets, though the left-side drop-target bank isn't as critical as it should be to gameplay.
You want to talk about art? Fathom is perhaps the most beautiful of all classic Bally's, and definitely up there in any age or era of games in terms of beauty. Use of color and design and integration of the playfield and backglass are second to none. Centaur's art is very obviously unique, and badass artistically in a way very few other machines are. I love the look of Centaur, and I don't think its art ever gets old. Put these two side-by-side and you have bragging rights, and gameplay for miles and miles.