After using a Series 2 Tivo for a couple of years I recently switched to to a SA 8300HD that I am using with Time Warner in So Cal.
The first thing I noticed is that the Tivo, even on the highest quality settings, had a noticeable impact on visual/audio quality. The 8300HD, even on standard definition broadcasts had a much nicer picture.
The Tivo GUI is better/smoother than the Passport software running on my 8300HD but for what I want to do, just record shows and faster forward through commercials, the 8300HD works fine. The Tivo does have some extra bells and whistles, but I found them nearly useless. Plus, all their software updates slowed the GUI on my Tivo to the point that I had to go with something else. It was just too frustrating. However, I don't believe the newer Tivos have the same slowness problem.
I also wanted 2 tuners, the ability to record HD programs and be able to use pay-per-view and video-on-demand services so that pretty much left Tivo out of the loop. Another problem with Tivo is I believe you have to sign up for at least 1 year of their service. With the cable box you can try it for a month and return it if you don't like. If you pick it up yourself and install it, that means you are only out $9.
I can't speak to using a TV Tuner under Vista (or XP for that matter) as I have never done it.
I would say try the TW DVR first and see how you like it then go from there.
-KeithP