USB is fine for an HD tuner...
I've got a Mac Mini with both the TV Mini HD and an EyeTV Hybrid attached to it via USB. They captures flawless HD content on two channels at the same time. Capturing HD content doesn't take much more than the tuner; it's already an mpeg stream when the bits come off the air, so it's already compressed and encoded - all you have to do is push the bits to disk.
Playback, as they say, is a b*tch. You have to have enough CPU to decode the mpeg stream, and enough I/O bandwidth to push bits to the display. Elgato and Miglia both say you need at least a dual G5 or a Core Duo. I couldn't get reasonable playback on a Core Solo or a Powerbook G4 - it was, as described here, "glitchy". After I replaced the Core Solo with a 2.0 GHz Core Duo (and upgraded the ram from .5 to 1GB), it played back beautifully. Shows I'd captured prior to the upgrade also played back beautifully.
Live tv is also gorgeous (I've got the mini tied to the DVI port on a 52" RCA 1080i rear projection monitor). If I try capturing on one tuner while watching live tv on another, the results aren't quite perfect, though I'm still trying to decide whether this is because the signal was flaky that night or that's just asking to much of the system.
All this is OTA. Getting a reasonable signal required installing an outdoor antenna. When I added the Hybrid, I put in a splitter and the weaker channels reported "Encrypted" on it and "Not available" on the Mini HD. Adding an inline amplifier from Radio Shack solved that problem.
To answer the OPs question, neither of these are suitable for getting HD content off of another device (thought they make a better DVR than anything short of a Series 2 Tivo). To do that, you need a device that will output bits to a data port or ATSC.
