Liszt.
I love both though, however Liszt is my favourite composer. As a pianist I think that Liszt simply destroys Rachmaninoff in solo piano achievement, although Rach's piano concerti surpass Liszt's (3 is my favourite piano concerto by anyone, and 2 is up there).
My favourite composition by Rachmaninoff is 'Isle of the Dead,' and his orchestration on the whole was better than Liszt's (although Liszt did improve after the 60s). However, Liszt was very influental as an orchestral composer and if not for him pieces like the Isle of the Dead probably wouldn't exist today. Rachmaninoff was a huge fan of some of Liszt's tone poems and also the Dante Symphony. Personally, I like Les Preludes as much as Isle of the Dead, and also I like both of Liszt's symphonies better than any of Rach's.
I prefer Christus to Vespers, and prefer Liszt as a choral composer in general.
Overall, while I love them both, Liszt gets the edge by quite a wide margin. The fact that Liszt was a far more influential and important composer helps his cause, too.