FWIW, I have a replaytv 4K and love it. I bought the 80gb model because it had a $100 discount, making it the best value per dollar of all their models. The thing I really like about it is the network connection. 3rd parties have already developed a plethora of utilities to work with it over the network. I am able to save standard mpeg files off the replay to my PC's hard disk with ease as well as make my PC look like another 4K and have the replaytv play video from it. Every day the tools get nicer and more functional.
I am sure that once the tivo can do ethernet over its usb ports, similar tools will pop up for the tivo. But, one thing the tivo won't be able to do is match the replay's network speed - I am able to pull the video off my replay at 2.5-3MB/sec over a 100BT connection, because USB 1.1 is limited to 12Mbits/sec you will never see much more than 1MB/sec off the tivo and probably less than that due to the various overheads.
BTW, the video hardware in the replay 4k is capable of doing HDTV level output. Right now it will only do 480p max, and sonic blue has never said anything about doing better, but the tivo2 doesn't have anything better than s-video output so no matter what, it will never do even 480p. It is a small thing, but for someone with a HDTV it makes a difference.
PS, I am one of those weirdos with broadband but no land-line phone. I use a cell for all my phone needs and broadband for all my internet needs, so the replay worked out just great for me.
PPS, neither is perfect - to do that would mean at least dual-tuners and support for recording HDTV signals too.