Liszt is INCOMPARABLY the greater composer of the two, Chopin's being a marvellous and innovative talent for creating memorable melodies, fortuitous harmonies and advanced figurations, but Liszt's welding all three of these to an (I would say) unerring sense of the numinous and (on occasion) the diabolic, that lifts every bar of his music (barring a few trifles) way above the ultimately rather mundane level of most of the Polish genius's output.
Only, I think, the finale of the 2nd Sonata and a mere handful of the etudes and preludes come even close to the otherworldly fiery fury of even Liszt's moderately successful creations -- the songs, the Consolations, many of the operatic fantasies -- let alone the Ballades, the Legendes, the Sonata...
True, no second Chopin has ever been born, and (arguably) no one writing after Chopin -- including Liszt -- wrote outside of Chopin's influence. But even if NO ONE writing after Liszt would have written with any knowledge or acknowledgement of Liszt's influence, Liszt's achievement would still remain a standard and a benchmark for every bar of music composed after, say, 1845, in a way that Chopin's would not.
What is this 'greatness', anyway, that, by comparing Chopin to Liszt, we are attempting to define? Surely, not the capacity to please. Nor a matter of their influence on later composers or instrumentalists. Nor, the ability to create 'good' music -- which it should go without saying is common to both Chopin and Liszt ... and (arguably) to Lennon & McCartney as well. When we talk about greatness, we talk about this inexplicable quality that makes a person and his/her achievement (when we talk about greatness, as opposed to mere 'quality', these two cannot be separated from one another: we cannot think of Kant's or Nietzsche's or Wittgenstein's philosophy without taking into account their persons -- but we CAN consider the merits of Heidegger's or Derrida's thought without considering Heidegger or Derrida) stand out as non-pareil. I can imagine a second Chopin. I cannot imagine a second Liszt. Contemporary audiences that considered Liszt a god weren't far wrong. Greatness inspires religion, rather than mere appreciation or admiration. And religion inspires dogma, rather than discussion. And dogma has it that there can be but ONE... etc.
Well, for my money, that ONE is Liszt, not Chopin. Amen.