They are both good systems, TiVo reaquires you to pay a monthly fee ($12 if i'm not mistaken) for the 'program guide' service. If you stop paying, then you have a paperweight on your hands. Apparently there is a package for something like $300 that you can pay all in one shot and get 'lifetime' updates- but i'm not so sure about that- if TiVo goes belly up then your stuck with no recorder...
Replay on the other hand, is more of an investment, BUT you dont have to deal with the whole pay for your guide shit.
If I were getting one, I think I would go for Replay, just because with the way the economy is now, and all these tech companies dropping like flies, I really dont need to go out and drop $300 on a box that in a few months might be as useful as a door stop...
later
Bern