With HD recordings you would need lots of room. The 160GB only gives you 20 HD hrs, and a 250GB only gives you about 40 HD hours? Therefore, With a lot of Tivo would keep deleting shows before you have a chance to watch them.
I have upgraded my S3 to 1TB and it has a lot of recordings, up to 150HD Hrs.
Also, it looks like the 250GB is from a TCD648250B and the 160GB is from a TCD652160.
You cannot go smaller than the original 160GB in the 652, nor 250GB in the 648. Also the software in the drive must match the Tivo box for them to work.
Cost wise, a 1TB cost around $50 and a 2TB is under $100.
Series 2 (PLS, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "652")
Roamio! Oh, wait, Its Juliet!
OTA! OTA! OTA!
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!