Patrick Coston's web site: TiVo and ReplayTV Comparison

This Compares the original Tivo to the original Replay TV.

I owned both when they first came out. This info may not be accurate for new models.

TiVo has Frame by Frame and Slow motion, Replay does not
TiVo shows a progress indicator when fast forwarding, Replay TV does not
TiVo's remote is smaller and easier to use
TiVo has a Thumbs-Up and Thumbs-Down feature which helps TiVo learn what kinds of shows you like and don't like. It can then suggest other shows you might like (so far TiVo has suggested quite a few shows, but I think it needs to learn more about me since it obviously thinks I enjoy movies from the 1950's which I don't)
TiVo rewinds a few seconds when you hit play after you've been fast forwarding so you can start playing where you intended and don't have to rewind or try to predict when to hit play. Replay TV does not do this. so it can suggest shows to watch, Replay TV does not
TiVo has something called Network Showcases where many of the major Networks like HBO, Cinemax, NBC, CNBC, display lists of shows they want to showcase. It's a quick easy way to find some really good programming. Replay TV has nothing like this.
TiVo has something called Tivo-lution Magazine where they the showcase the special shows for the week. Replay TV has nothing like this.
TiVo allows you to search for the shows you want to record. ReplayTV has not show search. You have to scroll through the TV show grid and find the show you want, and then mark it for recording.

Replay TV has a power switch, TiVo does not. You have to unplug it to "reboot" it.
Replay TV cost more but has no monthly subscription like TiVo
Replay TV's guide is a standard grid, TiVo uses (explain)
Replay TV has 5 Fast Forward and Rewind Speeds (1.5x, 2x, 4x, 16x and 20x), TiVo only has 3 and does not indicate the speed
Replay TV will have ads, Tivo will not
Replay TV can skip ahead 30 seconds to skip commercials, TiVo does not
Replay TV has an instant replay feature which jumps back about 8 seconds, TiVo does not
Replay TV has themes (explain), TiVo does not
Replay TV has smoother Fast Forward and Rewind than TiVo
Replay TV is good at telling you how much time you have left on your hard-drive. TiVo does not do this
Replay TV caches all shows when you pause while viewing live TV. Tivo only caches the current show and shows that follow. Tivo will not cache previous shows even if the new show just started.

Tivo has the concept of Season Pass which allows you to record a show every time it is shown. If you got a season pass to Frasier, it would record the new episodes as well as the syndicated repeats. ReplayTV does not have the concept of a season pass. You simply find the show you want to record in the TV guide grid and mark it as either record this one episode, or record episode each week. If you marked the new Frasier episode as record each week, ReplayTV would not record the syndicated repeats. You would have to mark those for recording separately. In my opinion, each has a feature the other needs. It would be nice to have the option to mark a show as either 1. Record show once (both TiVo and ReplayTV) 2. Record show each week (the ReplayTV way) 3. Record show whenever it's on (the TiVo way)
TiVo's remote takes 2 AA batteries, Replay's takes 4 AAA's batteries

Cannot record more than one show at a time
Not good for long term storage. Hard drive space is still limited and expensive. Once 100 gigabyte hard-drives become cheap enough, then this may on option.
Both require a remote control to operate. Make sure you have spare batteries on hand because once your remote dies, you can't use them.
Sometimes TiVo and ReplayTV will cut off the start and/or end of your show but about 10 or 20 seconds. I would appreciate a little more of a buffer.

Beeper on remote in case you lose it. Without the remote, you're left with a door stop. I lose the remote about once a week.
Log which reports shows that didn't get recorded because there wasn't enough space so you can see which shows didn't get recorded
Ability to record multiple shows at the same time
Ability to adjust priority of Season Passes to have the conflict resolution you desire
More intelligent scheduling for conflict resolution. Some shows are on at multiple times. When there is a conflict, have the ability to reschedule show records to other times.
When shows are deleted from the schedule before they are recorded, automatically fill in that empty slot with other shows that were previously unable to record
Ability to edit video. Save a snippet, delete a porton (like commercials) or combine records into one
Ability to organize shows in sub-directories
Ability to easily expand hard-drive space
Ability to share shows peer-to-peer with other DVR users
Report on how much disk space is in use and how much is left. Perhaps display in time based on different quality settings
Ability to archive shows to exeternal peripheral, perhaps a PC
Ability to Suspend a Season Pass, instead of deleting it
Auto-Variable Quality: Record at low quality when there is not much change, Medium Quality when some changes and High Quality when a lot of change.

Both TiVo and Replay TV delay the signal you see from regular TV (about X seconds). TiVo's delay is a split second longer than Replay TV's.

More info: ReplayTV and TiVo FAQ (updated November 5, 2000)