New to the XBMC world thinking of getting a Raspberry Pi

I am new to the XBMC world but have ran it on my iMac with some success. I am thinking of getting a Raspberry Pi instead for its portability and am looking at these two on eBay (not affiliated with any of these sellers). I am clueless on which to get so which would you guys recommend and why? Also how stable are they, curious if the hardware stays up to date easily as far as the XBMC or it's a thing where you buy it now--and a few months down the line everything is outdated? Do they last long?

Also are they really powered by just USB and have wireless Wifi, could I do wired ethernet as well? What is your experience with them?

http://www.ebay.com/itm/Raspberry-Pi-Ult...1e7a7d7c0e

http://www.ebay.com/itm/Raspberry-Pi-512...3f23f89233


Thanks!