In that case, technically, Art Tatum is harder
I would say that if you play Liszt, playing jazz and improvising would become "easier"
Why?
Liszt improvised over theme's. His writing is very pianistic and if you spend time on his music, it will never become impossible for you to play one of his pieces. He is more "virtuoso" and has less music in my opinion (now I am not putting down his music, it's just for you to see that his music, most of it, is full of 1,000,000 notes that if you eliminate them than you would really listen more music rather than a burst of notes) Unlike other composers like Chopin, you play Chopin for 5 years and it still doesn't sound the way it should be even you have the technique.
Liszt sounds harder than what it really is, the only problem is that if you don't have the technique you won't get there.
Now, with Tatum, the disciplines both men had are completely different.
I would say that Tatum is the Liszt in Jazz, I guess everyone would agree..
Now if you get the two composers, one studies counterpoint, harmony, works of his teacher, and some etudes providing technique, he creates another work from another (Paganini, MOzart, Bach, etcetc Well.. arrangements..) etcetc. The other one studies improvisation, plays folk songs and then they teach him reharmonization, chord types, scales types (dorian, phrygian, lydian, mixolydian, etcetc) chord voicings, comping, accompaniment, etc.
My answer to your question would be, as I stated above, that both require dexterity and technical mastery. They both put their effort and strived during their lives to become what they've become. No one wins, they are both great and their work require a lot of dexterity, technique and knowledge.
Ok.. this is only a suggestion and I would you people to share your thoughts with me: Someone may kill me if I said that it would be a waste of time if you tried to play an arrangement of any of these composers.. why not trying to do your own arrangement being inspired by this two great pianists? I mean, you would feel more comfortable if you are the one creating your own.