If the cable company is offering you a dual tunner HD DVR for $5 bucks a month that would be my choice (even though I'm a ReplayTV fan). If not, I'd go with Replay for all the reason siCorpRoadie states.

TiVo has a cute user interface, but feature for feature Replay TV is better, provides all its functionality out-of-the box with no additional cost (networking, watch/record shows on other boxes in your house, 480p output, and more). Here's a comparison list Replay comparison.

There are presently no standalone DVR's the have dual tunners and HD, so there isn't a direct one-to-one comparison of STB DVR and Standalone DVR.