With the two options you posted, I would go with the Boxee. Too many people are saying they too are going with a media streamer because with a HTPC there is constant tweaking and tuning to keep it working properly, and because of the complexity of it, it is not wife-approved. There is also the cost factor. From what I have read, the Boxee is going to cost around $200, whereas a good HTPC could cost three times that.
The one drawback could be Boxee's ability to stream full bitrate 1080p or 720p rips. Nobody seems to know for sure how fast it is going to be over a network. Most players that have an internal HDD or external usb drive will play them fine if the video is stored there.
Another option would be to go with either a Dune Base or Dune Prime, selling for around $400 and $500 respectively. I don't know what the UI looks like (I can't afford one right now), but from reports it is top-of-the-line. Evidently it will stream high bitrate files over a good network, as well as passthru hi-def audio, like TrueHD and DTSHD-MA, to a receiver. Don't quote me, but I think I read that the Dune players have gigabit networking.
My recommendation would be to look around some more, do a bit more research. If need be you could buy an inexpensive unit like the WDTV Live that does a decent job (in my opinion) until the perfect, for you, player comes along. The WDTV Live is a bit finicky with the network setup, but will stream SD video over the network all day long, and play hi-def from the an external HDD nearly flawlessly.