Get a Roku if your primary interest is Netflix, IMO. It's tiny, low-power, has an awesomely simple remote, and seems to work better than any of the other Netflix streaming methods I've tried (PC, TiVo, 360). I usually choose my bedroom Roku over my living room TiVo Series3 just because it's simpler and more reliable.
I also wouldn't want to power up a game console and go through its UI every time I watched something, and I'd be annoyed about paying an additional subscription fee (Xbox Live Gold) to get Netflix access. If you'll do a decent amount of gaming on it that takes it out to an apples vs oranges comparison, though. Only you can make that call.
I just posted a link to an Endadget Netflix shootout between the Roku, TiVo, 360, and a couple Netflix-enabled Blu-Ray players in
the OOT Netflix Watch Instantly thread.