Either wait for the HD Tivo, or choose between recording/timeshifting and HD.
Many people, once they are used to PVR/DVR, never want to go back. Many, also, once they see HD, are hooked. I have 4 Replays, but still watch Prime Time and D* HD in HD, and then other timeshifted things on the Replays.
Since you aren't used to a Tivo yet, I'd get a HD STB now, and then a HD Tivo when it's out and stable (that may take 6 months, seriously).
Hope that helps!
(And your subject could have been better put as " Choose HD STB or SD Tivo?")