I would just like to chime in a bit.
If you use the XBOX, you are stuck with either a Gold account for Netflix or Playon. With Playon, you will NOT
get HD streams from Netflix. You will NOT
get FF functionality (past the end of the buffer) like you do with the devices. PlayOn is more like streaming a video where you can only watch what you've downloaded. You can't jump to (for example) 1 hour in.
If you use the native players, you can FF and Rewind just like if it was a disc (no chapter skip, but you can FF up to 120x I think, so it's pretty speedy), and then it just starts from there.
I cannot in good conscience recommend PlayOn for Netflix streaming given the lack of HD and decent controls while watching the movie.
Given that, I would recommend getting a PS3. It does media streaming just fine, PlayOn works fine with it, and you can also watch Netflix through a native player getting full benefits of the streaming offerings. Plus I believe you can plug a 1TB FAT32 drive right into the PS3 (I think...not sure)
PS3 doesn't work with Windows Media Center though, but there are plenty of other options.
I just don't see why you'd get an XBOX + PlayOn for this, when you could get a PS3 + PlayOn and basically have media-streaming from the PC and Hulu, plus the added benefit of a built-in Netflix streamer (I say built-in because even though it does require a disc right now, it's still a "native" streamer in the fact it gives you full FF/Rewind/Browse functionality and HD streams).
That being said, I use my XBOX + PlayOn, but I have a gold account for gaming, so it's not like I feel I'm paying extra for Netflix since I already had the gold account. I'd use my PS3 for it if it turned on with my remote (Yes, I am that lazy that I hate to get off the couch to turn it on and put in the disc)
Finally, one thing to keep in mind. The XBOX has Zune 1080p smooth streaming with 5.1 sound. This works amazing (see my impressions here: http://www.avsforum.com/avs-vb/showthread.php?t=1191057
). But the PS3 has the PSN which is ok for the most part.
Honestly, no matter what you decide, you're going to be making some trade-offs....but if your primary focus is Netflix + Hulu (without having to pay extra for Netflix or Hulu+), I would go with the PS3+PlayOn solution over the XBOX+PlayOn solution. You'll get the same Hulu experience, but a much better Netflix experience.