I've used both and vastly prefer TiVo. I could explain why but this comparison does it for me:

http://pvrcompare.com/tivoreplayframe.html

Edit: It appears that Replay finally got rid of the terrible guaranteed/non-guaranteed model and does it's space management more like TiVo now. That's a definite improvement but it still doesn't have a To-Do list or exposed priority list for recurring recordings. I consider these absolute necessities, especially on a single tuner recorder.