I own both the ATV2 and Roku XD (wireless only). For what it's worth here are my observations of the pros and cons of both devices.
ATV2 - Works great within the Apple ecosystem, it's main strength is Airplay coupled with it's superior wireless radio (by which I mean it is dual band N, and can see and operate in the 5 ghz spec) Netflix is, IMHO a bit nicer and polished than on the Roku or just about any other device i've seen. IF you have an Ipad (also dual band) or maybe an Iphone (not dual band) streaming wireless HD content using Air Video Server / Client is fantastic. Air Video ( http://www.inmethod.com/air-video/download.html
) uses FFMEPEG to transcode just about every video codec into .mp4 on the fly at a bitrate you define, depending on your wireless networks efficacy. No jail-breaks or mods required, though a good wireless router and network optimization is key to best performance. So, if you want a good _wireless_ streamer and have at the very least an Iphone (Ipad is MUCH better for this) then the ATV is worth every penny. The biggest complaint I have with the ATV is the remote. The tiny little thing is an escape artist with it's wafer-thin profile and rounded edges it can slip into the most unlikely spots (be sure to wear underwear while operating your ATV.)
Roku XD- I just can't recommend this device for HD wireless streaming, though as someone pointed out, the wired version could be much better. Plex is an acquired taste, if you have a nice CAT5e run to the Roku, it could be your cats meow, but for me it just feels a bit clunky and afterthought. Channels, well, such as they are. Know the "secret code" and add it into your Roku via Rokus' website. I personally do not like to have to use a _website_ that is not running on my own hardware to "add" anything not offered through the "channel store." In fact, be prepared with a laptop or tablet when you setup your Roku, as you have to enter activation codes to even get the thing up and running. It feels a bit too invasive for my taste. The "secret channels" that I have added are made by 3rd parties and act more as stream aggregators and are pretty rough, though some might find the process fun, in that there is a hunt for codes, and you might feel a little thrill getting them to work. Video and Audio quality are good if not great - if you have the bandwidth available (which on the Roku XD with no eth0 and no 5ghz reception) you probably don't. If you just want bang-for-buck for a basic streamer for netflix and hulu+ then hey, this is not a bad option.